If mana was a fixed resource that always served as a limit to the group being able to succeed, then I would agree with this. But it doesn’t. Every spec except for possibly resto shaman (I haven’t played it enough recently to remember) has ways to keep from going out of mana even over the course of an M+ dungeon where the tank never stops pulling. The idea that mana is a disservice to a fast paced game is simply off the mark.
I mean the top two healers in M+ and 2 of the top 3 healers in raid are resto druid and shaman, both highly dependent on their mana. If mana functioned the way it did back in the day where it effectively always went down, those two specs wouldn’t see the amount of success it does.
You’re missing my point here. The healers like preservation evoker and holy paladin and mistweaver monk are designed around mana not being the main limiting factor to their ability to keep a group alive. They each have other resources, some fairly unconventional, as the thing the healer needs to keep track of in order to succeed. But that can be just as much a way to add variety to the different healers rather than an indictment of mana as a resource.
Nor can I, and frankly I don’t want to see that return. But Blizzard has shown that mana can exist as a constraint on healing without relying on the existence of a mana return stat to prevent players from going OOM during a raid fight/dungeon.