4E narrowed the band of power levels considerably. There were a lot less 'world shattering powerful magic spells' and more 'slightly upgraded or degraded fireball'. The powers were very similar between classes, giving everything a generic and non-iconic feel. That makes it hard to have a high fantasy adventure with the feel of Lord of the Rings ...
And here I'm going to tell you that you've contradicted yourself. If you read Lord of the Rings there are almost
no "world shattering powerful magic spells". Indeed there are almost no magic spells
period cast by Gandalf (there are reasons he's notoriously
a fifth level magic user)
World shattering powers in the hands of PCs belong in some genres - if you want to play
Dragonball Zeta or
Marvel Superheroes (and I'm not calling either a bad thing) then world-shattering powers are part of the genre. But I'm struggling to think of a single work of high fantasy that's not D&D-derived fiction where world-shattering powers are the norm. Even if we go to Harry Potter levels of ubiquitous magic there isn't that much they do that's world shattering.
Lord of the Rings in particular is right there as a 4e game. It's a game where eight of the nine members of the Fellowship are not spellcasters (or in Aragorn's case is possibly a ritual caster) but you can actually distinguish how they fight mechanically rather than giving them interchangeable character sheets - it's especially the case for the films rather than the books.
Meanwhile if I want to give everyone a generic, non-iconic feel I make them TSR-era fighters where there is almost
no difference in the combat abilities. Rather than have fighters with Come And Get It and rangers able to blacken the air with arrows.
The entire Daily / encounter / at-will structure.
Dailies and at wills are pure D&D. You recover your spells on a short rest and melee attacks are at will.
The thing 4e changed was to become a game that better models both real life and fantasy fiction by having fighters who paced themselves rather than were untiring robots who always used the same attack numbers irrespective of how long the fight had been going on for and how many previous fights they'd been. And that it became a game that better models high fantasy by allowing the casters to cast minor attack spells all day - unless they were one of the few sword & spell classes.
If you want high fantasy as ever 4e models it better than all previous D&Ds. And it does it in part by sharing the iconic parts. 5e to its credit kept this structure although it made short rests too long.
The flip side of removing vancian magic - the inclusion of rituals.
Which is one of the many, many things which makes 4e
the D&D that does high fantasy. And previous editions of D&D only really suitable for "D&D Fantasy". Magical rituals are a thing - you don't have all wizards casting almost all spells at combat speed. 5e keeps rituals - and a damn good thing too.
This I'll give you. 4e monsters aren't just bags of hit points that are more or less mechanically interchangeable, behave like wizards with prosthetic foreheads, or make incredibly tedious claw/claw/bite/wing buffet/wing buffet/tail slap attack combinations.
Introducing new classes like Invoker, Avenger, Warden, etc...
I think that literally every edition of D&D except 5e has introduced new classes like Thief, Ranger, Assassin, Illusionist, Cavalier, Warlock, Artificer, etc. This is neither more nor less than a claim that 4e is weird because it does what all previous editions of D&D did.
The mathematical structure where everything went up by 1.
Again this is part of what makes 4e better at high fantasy than other editions.
So what you're saying is that 4e changed D&D by being very much more suited to high fantasy settings like Lord of the Rings where you can have large battles against dozens of orcs and don't need to track the hit points of each one.
I could go on for a long time about how 4E was an abrupt change from prior editions. I don't need to, however, as there are countless threads on it - especially from the early days of 4E. You can do a Google search and read all night about it.
And literally every single change you've mentioned is something that makes 4e better at modelling high fantasy and common works of fantasy fiction than other forms of D&D.
I have 10 years of threads that disagree.
And as demonstrated above a lot of your examples are completely risible. I'm pretty sure that if I were to search in the right places I could find more than 10 years of arguments for a flat earth. Now your position has more merit than a flat earth - but just saying "I have threads where this was asserted" is not an argument.
Your characterization of the games of grognards is almost entirely opposite of my experience of 40 years of D&D - which has spanned a wide section of DMs, from those over 20 years younger than me, to ones 20 years older. There are some DMs that fit your description, but the majority are creative and open....and this is where our experiences differ. This is one of the key tests I have when talking to a new DM to determine what I want to play in their games. I ask them, "If I played a [XXX] warlock, how much would my patron impact the game?" Then I listen to how they describe the interaction. If they're excited and passionate about it, that is a great game to play a Warlock, Cleric, Paladin, or a PC with strong loyalties to an NPC. These are the DMs that see a warlock as RPG gold and give you a great story in addition to fun combats. These are the games where I dig out the character ideas with a 10 page outline of a backstory that we go over, tweak to fit their game, and then spend 2 years appreciating the narrative that grows from those seeds.
On a sidenote here's something you can thank 4e for. The warlock class was technically introduced in 3.X - but the warlock was supposed to inherit those abilities. By talking about warlock patrons you are injecting pure undiluted 4e fluff into your game. This is yet another reason 4e is far,
far better at high fantasy than all previous editions of D&D. For that matter the vastly superior 5e approach to Paladins over previous editions where paladins hold to an ideal and don't fall (but can be fallen) rather than legalistic codes of conduct based on alignment is also both pure 4e and much more in line with high fantasy than all previous editions. I suppose 4e really did change things by bringing that sort of interaction front and center.
In my experience, I find more excitement and drive in older DMs than I do in younger DMs.
In my experience there's significant survivor bias.
I've met a few young DMs that are trying to emulate Mercer's style, and I always let them know that I highly appreciate what they're trying to do. However, I know a lot more older DMs that are very skilled at this style of game - and most of them are not emulating Mercer - they're practicing the art they've practiced for 30+ years (though I do encourage every DM to spend a little time listening to Critical Role and critically thinking about what Mercer does that works that the DM in question does not do - he is an amazing DM and being able to study his style is a blessing).
I encourage this - and DMs to work out what Matt Mercer doesn't do so well. He may be the world's best at what he does but isn't great at absolutely everything.
Also, I have a series of house rules. The idea of house rules seems to be disparaged quite a bit in this thread.
Where? But what's disparaged is saying "My house rules do it therefore it's part of the game" - no it isn't; it's part of your vision of the game.
They give a benefit to flanking that is less useful than advantage, and strategically reintroduces the idea of lock down maneuvers (without making it overpowered).
Oh hey! You're trying to reintroduce things that were only done well in 4e

(Removing Dex bonus from AC!)
However, when they go out into the world, they find monsters that are not in any 5E book, they encounter spells that they've never seen and are excited to learn about, and they never know what to expect.
And once more you're trying to introduce elements of 4e into your game. Monsters that aren't in any book? That's
why 4e monster design makes it so easy to create effective and balanced monsters that are actually mechanically distinctive. Spells they've never seen? 4e is the
only edition not to rely extremely heavily on cookie cutter spells.
They have to look for context clues to figure out if they're fighting a monstrosity that uses brute force, an aberration with supernatural abilities, or a few creature with trickery and magic.
Once more you seem to want to do things 4e leaves all other editions in the dust for.
They don't just hear half of the description and say, "Ah, grells. I know how to fight them."
Again 4e leaves all other editions in the dust here. Monster roles and monster powers aren't anything like as cookie cutter or even made up of either their number of hit dice and AC or a network of feats.
I find that players really enjoy the experience of the unknown when it is dynamically introduced.
Again, welcome to something 4e does better than any other edition.
Friend - I don't have the time to go through ad recreate the 10 years of commentary on this topic. It stands for itself. My use of fireball was one example of many, intended to reference how so many abilities were incredibly similar ad cookie cutter. I'll let the 10 years of discussion on this topic stand and accept that you disagree.
If you want similar and cookie cutter then how about a game where all melee combatants make their basic attacks using exactly the same mechanics. Rather than the sword and board fighter using
Tide of Iron to drive the enemies back as one of their At Will attacks while the rogue gets to slip round the edges of the combat. 5e does a
little of this - but where you say "so many abilities were incredibly similar" this compares to previous editions where combat abilities were
literally identical except what you added to the dice.
For that matter let's look at specialist wizards. In editions prior to 4e (with a couple of honourable but dead end exceptions like the Illusionist from 1e and the Dread Necromancer from 3.5) a specialist wizard would cast the exact same spells the exact same way as a different specialist except with maybe a small bonus to a saving throw and maybe some spells you couldn't prepare that they
didn't prepare.
In 4e, and 5e following in its footsteps a specialist wizard is literally better at casting spells they specialise in and gets bonuses to those spells. Yes a fireball cast by a 5e evoker is incredibly similar to a fireball cast by a 5e illusionist, with the only difference being that the evoker has better control and can protect allies in that fireball. This is the sort of change introduced by 4e. And if you want to call this cookie cutter be my guest. But in editions
before 4e rather than the two fireballs being similar and, in your words cookie cutter, the fireballs would have been
literally identical. With the only difference being that the evoker had the chance to prepare a single extra fireball per day.
You are taking changes from literally identical to similar but meaningfully different and somehow claiming that similar but meaningfully different is more cookie cutter than literally identical.