Quite possibly, M+ is an adaptation of Guild Wars 2’s Fractals of the Mists, since that came out before M+ (2014 vs 2016).
I shouldn’t be doing Padgarre’s homework for him, but back in the day, classes and specs had much more of a niche role. When the game is designed around 25 - 40 players raiding, certain assumptions about the which specs will be played can be made, and encounters designed around that.
But when Blizzard decided to make 10-man raiding a thing (due to how popular Kara was), that assumption could no longer be made, so they had to start giving many classes tools they previously did not have. Blizzard billed this as “bring the player, not the class” at the time, but it lead to complaints about “class homogenization”.
With M+, this “homogenization” became more pronounced because the groups are smaller; if certain classes do not have the tools to deal with the current meta, then you get what we have now, where Resto Shaman are seen as more useful than other healers as a result of their spec kit.
Having said all that, I don’t think classes being able to “do more” is bad; I played Ret during BC and it was awful, on account that unless you were tank healing or off-tanking, you were basically useless.
Also, I’m 99.9% sure that when players complain about the game being worse now than before M+, it’s not actually M+ that made the game worse, it’s just an easy scapegoat because of its popularity.
(It’s kind of like how players complained the game was better before the ability pruning of WoD – and surely naught to do with WoD’s lack of content – and yet, here we are with many un-pruned abilities that don’t really have a reason to exist except to try to placate players who were upset…and I’d wager the un-pruning didn’t even bring back or placate the complainers anyway.)