All I can say is that the game clearly breaks down if you use the provided options in the PHB: multiclassing and feats. Also all the cool magic items in the DMG? Forget them if you want to use monsters as-is.
Sure all editions of D&D has become wonky at high levels, but after playing the game for twenty years it has never felt so... empty... like playing a game on easy mode.
Of course any good DM can rectify this. But that does not change the basic fact that the monsters of MM and the encounters of published adventures are very very easy, probably way too easy for any party that knows its stuff, which they presumably do given they have survived to reach high levels. In other words, yes you
can fix it, but you still
need to fix it (as opposed to being able to run encounters more or less as-is).
The monsters simply aren't equipped to cope with the player characters, that have more tricks than arguably ever. (Sure high-level wizards have been toned down, but how does that help when everybody in the party can toy with the opposition). They're far too naively designed, with very few built-in ways to work around or negate class abilities and spells.
One area is especially underserved: the classic BBEG, facing a whole party on his own. You either have to overlevel the BBEG to ridiculous amounts (like perhaps a CR ten steps higher than the APL, or even more!) or you have to add, well, adds.
The game (and now I mean "combat") works much better when there are at least as many bad guys in an encounter as heroes.
I can totally see why they focus on levels 1-10. They have simply gotten the feedback that their high-level efforts does not work. Like. At. All. So why bother, when every DM will need to pretty much redesign encounters from scratch?
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I prefer to look it like this:
If we instead say 5th edition is Basic Dungeons & Dragons, perfectly suitable to welcome new players, and to play low- and medium-level content (where most people prefer to be anyway), it is
an excellent edition.
All us grognards need now is an Advanced Dungeons & Dragons, with a bit more crunch-depth (meaning more ways to make your character mechanically richer than just mc & feats), but mostly with expert monster design in a completely new Monster Manual, so classic solo threats at every level (like a Hydra, or a Banshee, or the Gorgon, or a Marilith, or... you get the point, not just dragons)...
actually challenge a party of five even if players are experiences gamers and use the provided crunchy bits and have feats, several classes, magic items and more
My ideal, then, is an AD&D edition that remains 1000% compatible with the existing game, but with a brand new PHB, DMG and MM:
AD&D PHB: this is the existing book that needs the least work. Move the Epic Boons stuff into the AD&D PHB, print the Revised Ranger, add a new class to supplement Sorcerer, and replace perhaps the weakest dozen of spells with a new batch and I would basically be happy. (Also, fire the old index guy and hire somebody who has seen a good index before, and let him or her add class info to each spell description entry)
AD&D DMG: replace all the world-building and gamesmastering and encounter/challenge stuff, since that's worthless to experienced DMs anyway, and fill up the freed space with many new magic items. Ideally, of course, complete with a optional variant utility-based pricing system to replace the Rarity-based ratings (that remain for backwars compatibility). One thing I would want is for D&D to finally embrace "partial results" so that a BBEG can fail a Banishment or Hold Monster or whatever, but still not be outright crippled. Legendary Saves, sure, but it's crude, simple and not especially satisfying. Much better if each party spellcaster needs three "strikes" before his or her spell "takes", giving progressive bonuses (and descriptions) along the way.
AD&D MM: okay, so each humanoid gets several variants all the way up to CR 5. The Bounded Accuracy claim was that you could use Orcs or Dwarves even at high level, just in greater numbers. This is balderdash in a game with spells that autokills mooks with less than 25 hp (Spirit Guardians). Everyone from goblins to grimlocks to wood elves get specific spellcaster entries, since you simply can't touch a party with muscle alone (unless said muscle is something like a Frost Giant, of course).
Then we add a new layer to the game: the Legendary Solo. This critter is designed to challenge a whole party on its own, no mooks expected or assumed. The hit point calculations of the monster design is woefully underpowered, and the designer of THIS book knows that even at medium level individual player characters can push out in excess of 100 damage per round, easy. Any CR 10-12 Legendary Solo thus needs to sport
at least 500 hit points, assuming the "Let's just kill it" strategy isn't meant to be the straightforward solution. At the same time, unlike a severely overleveled bosscreature, its attacks wont singleshoot a PC, so the combat can actually
work.