Come on! The Tank, Healer, Damage dealer trio was effectively codified into D&D from the first publication.
Not exactly. At least, not within one combat.
Combat before 4e was a very fast thing. 3e had its infamous rocket tag, but even 2e and 1e had fragile characters and big, swingy effects (often spells) that could make or break a fight in a single round. And healing happened between fights, not during them. There wouldn't be a kind of ebb and flow to a fight - it's over ASAP.
Early design even encouraged this through treasure-as-XP. Fights were only risk, no reward.
We see in 5e a move back toward faster combats (though not nearly as binary as 3e and before could be, which is overall probably a good thing).
Because combat is only one part of the game. And it can be a relatively small part if the focus was on, say, dungeon exploration, or storytelling.
Like a lot of 4e's more controversial decisions, moving to a role-focused class design meant moving something that could be true for some subset of players and making it a major concern for every player.
D&D has often struggled with this tension of defining what its goals of play are. Tell a story? Raid a dungeon? Fight some monsters? Different tables had different weights, and they could all be playing the same game (and, of course, complaining about different elements of it).
If the main goal of play isn't "fight some monsters," (and it wasn't, for thousands of players) then combat roles are not a useful addition, because combat is not really
supposed to be a focus in those games.
The AEDU power structure also is an iteration of this same design choice of emphasizing combat.
For some tables, fighting some monsters IS the goal of play, and those tables were probably very happy with roles and consistent power structures. Issue being that even if the largest percentage of players have a goal of fighting some monsters (say, 40%), you might still have a huge number players whose goal is NOT that, and who will chafe at design decisions that emphasize that.
One of the things that 4e's bold choices highlighted was that the diversity of people with a vested interested in "playing D&D" is so large that it can be very difficult to build this particular game in a very focused way, because your focus is inevitably going to exclude big swaths of the player base.
Edit: might be worth pointing out that 5e's perspective on roles is much more resilient to different play styles. The roles are definitely there and different classes are better at different things, but it's not like you can't build a damage-focused Fighter or a healing Warlock or whatever. The roles aren't definitive, they're implied and suggested. I'm sure for the more combat-focused groups out there, this is a worse choice, but it's probably a better choice
for D&D, since D&D needs to serve many masters.
