Disagree. Both Ragnarok Online pvp and World of Warcraft arena had tank mage builds (discipline priest and priest/high priest) and they were incredibly fun to play. Especially early-ish discipline pvp was all about proactive absorption and then removing as many buffs and burning mana of the enemy.
Then of course blizzard did their usual blizzard shit.
I still think it’s okay to have high skill ceiling classes though. It’s one reason dota2 Invoker was so popular for example.
They could probably add one healer in a moba or team shooter that has a complicated but rewarding game play.