Couple things on this:
1) Brutes are always more potent when coupled with a Leader.
Sure. And 5e ogres are more potent when coupled with the 5e version of a leader. They're also plenty potent on their own.
But the more relevant thing is that points like this exhibit the fiddly bits pretty well. One of the things that really didn't mesh with my playstyle in 4e - on both sides of the table - was this focus on "build." If I have an encounter with ogres they won't be really significant unless I
also pair them with Options B, X, and Q? That's too fiddly for me. The decision to fight some ogres should carry a lot of impact on its own without 3-4 higher-order decisions also influencing whether or not I
really got the
real effect of some ogres.
2) I'm wondering if your GM (or you) used updated math Brutes or MM3/MV Brutes (or the mid/later Dungeon ones). Speaking of Red Dragons? Low paragon tier multi-headed Red Dragon Calastryx (Brute)? A Wizard will have mid-60s HPs when Calastryx will be encountered;
a) Attacks on Init 30, 20, 10 (and 40 when Bloodied)
b) Does 28 damage with a Slide 3 rider (keeping you nice and close for furthing OMNOMNOMing...for sitting in her Breath Weapon conflagration rider = 10 fire damage/turn...better have a teleport or a Defender!).
c) Each of her 3 heads tracks Breath Weapon recharge and Rip and Tear usage on their own (so basically a possible 3-4 Breath Weapons or Multi-attacks on one combat turn...yikes).
That is a Brute. And that Wizard is lunch if things don't go right.
I DMed and played throughout 4e's lifespan, so I'm sure there was a mixed bag of MM3/MV critters as well as earlier critters. That +25% damage thing comes directly from the post-MM3 math.
If it takes a monster three separate steps to be a Brute, including 4-5 attacks, sliding, an ongoing effect, and tracking three different heads at once, it's making me think WAAAAAAAAAY too much about it. 5e ogres do it in one: dealing
massive damage with a friggin' club. Everything else is unnecessary cruft. That's what I'd call effective, elegant design, a joy to use, across multiple levels and even at scale.
You actually picked the only dragon age that wasn't updated. Even if you would have picked a Young Dragon, things would have been different. Even still, while the non-updated Adults aren't remotely as nuanced/good as all the other dragons, they are still observably different from their kin in how each would manifest in play (which I thought was the premise?).
As I pointed out, I don't think the differences between the Adults in 4e are really all that MORE noticable than the distinction between Adults in 5e. The big thing a lot of players are still going to remember is: that one breathed lightning at us in a desert, that other one breathed fire at us in a volcano. They're about the same in terms of distinction from each other (ie: not very), 5e's just simpler about it.
Some pretty large pieces of the puzzle missing here.
a) The Opportunity Cost of subbing a 5e Dragon Multiattack for the Shove Action is way, way, way too steep. You better be ensured an auto-gib from fall death. First, you have to have a contest for it to work. Second, it is either 5 ft or prone, not both. Meanwhile, the 4e Blue Dragon is (i) doing a chunk of physical/lightning damage to you while (ii) automatically pushing you 3 squares and knocking you prone. Prone, which is a more punitive condition in 4e than it is in 5e.
That is a suite of melee riders built for a "mobile, sky-kiting, lightning/thunder hurler." Get out of melee and right back on the wing. And maybe drop someone off a cliff after you've NOMMED them.
The 5e dragon bites you, deals phyiscal/lightning damage to you, then trades one of its claws for a shove. The 4e dragon bites you and shoves you and knocks you down. This is the same thing in essence - the differences there are subtle and nuanced and so end up often being mostly irrelevant in practice IMXP. At least when compared to "that blue thing breathes lightning" and "that red thing breathes fire."
b) Yes, my guess is 5e's "Wing Attack" was pulled directly from the Blue Dragon "Wingclap." The thing is though that all dragon's generically have Wing Attack as a 2 LA costing ability that does the same thing regardless of dragon. The premise (I thought) was distinctive things across the dragons. The 4e Blue Dragon's Wingclap synergizes with its other unique abilities to create a distinct draconic experience (very distinct from the Red) at the table for both player and GM.
A big part of my point is that if you have to start talking about "synergies" you're starting to admit that the thing on its own isn't distinct. 5e dragons can be mobile when they need to be. I don't feel like it's a problem that this ability isn't reserved for certain
flavors of dragon, and forbidden to other flavors, because one dragon doing that and one dragon not doing that isn't really that big of a difference in play IMXP.
You also have to consider everything in context (especially (a) and (b) above). 4e's melee is passively-persistently "sticky" with lots of participants having active abilities to punish or restrict movement (see the updated Red Dragon that you didn't use for your argument - 3 * mid/high DC Grapple + Interrupt on any movement within 4 squares and you get smashed and come right back next to "good ole red"). Everyone isn't just a Skirmisher by default. Having resources to escape the passive/active melee vortex is absolutely central to being a functional and effective Skirmisher or (in this case) ranged Artillery.
Yes, more "builds," higher requirements for system mastery, more "you're doing it wrong," etc. All for very little effect when the players mostly remember the lightning vs. the fire.
c) and d). C is what it is. Stunned until the end of your next turn nearly the entire time (5 % of the time saved ends instead...not too complicated). And D is what it is. Not just a "different breath weapon." Its another long range, deadly kiting tool. The difference between Range 20 and Close Blast 5 is gigantic (as you know).
Adult Blues with their BW recharged are choosing between a breath weapon that's "a lot of damage to a single target vs. AC" or an area attack that's significant damage to a cluster of targets vs. Reflex. The areas and targeting and damage values don't lead to a lot of distinction in practice IMXP. It might as well not be there.
The Lair Effects (which I touted all through the playtest as the best 5e tech along with the Background Traits and a few other things that didn't make the cut) and Regional Effects are what make the 5e dragon's distinctive. Their basic chassis is entirely nondescript (save for Burrow vs Climb...which, given the fleeting fury of 5e combat, may...or very well may not...be featured for a sparing moment on-screen).
They each need a personalized suite of synergizing Legendary Actions.
Nondescript is fine. Just having a different breath weapon shape/damage is a significant distinction when combined with their fiction elements. They don't
need unique and particular powers that speak to specific combat roles to achieve distinction. They never really have needed that. The difference that everyone knows between red dragons and blue dragons is that one shoots fire and one shoots lightning, and adding to that is a lot of cruft for very little value.
Again, I'm cool with that being an opt-in element that 5e could certainly stand to add to its monsters. But I'm happy with 5e monsters as they are - straightforward - and if the option to make them more complex was available, I don't think I'd actually use that option much. It's not a thing I want or need from my monsters, so adding it to my games would be adding a lot of work for little reward. So I'm glad the base versions of these creatures aren't complex like that - I don't have to "build" my monster's identity out of 4-5 disparate parts assembled
just so. I can just say "my blue dragon breathes lightning," and it's fine.