Mana management in vanilla and TBC was downranking and healer rotations, none of which were particularily fun or interesting mechanics. It required no skill whatsoever to pick a correct rank spell or if you're healing or wanding to regain mana from spreadsheet or addon for a fight of x minutes long which guaranteed you'll never go OOM even if you press that button on every GCD. No matter how much healers whine or cry to get that back and they loved it would not have made a lick of difference because it was totally unbearable situation from game design point of view and a dead-end evolutionary path.
The idea between Cata's revamp where you had to think which button to press with variable incoming damage patterns worked great first few tiers and actually required some skill from healers, it just turned into mindless spam due to gear scaling later and today is more or less just smart heals, absorptions and raid cooldowns.
You can be sure there's gonna be another big revamp in next expansion too that tries to break the scaling problem, just like talent revamp required few iterations and expansions to get it right.