Well, healing in an action based game would require some workarounds, but I mean, healing works pretty well in Overwatch, for example. Tanking is already possible in that system, but again requires game makers to actually learn from their peers. Take Black Desert Online for example: Blocking in that game gives you a shield counter which maxes out at 100, and attacks will deplete that number. Work something like that, and a timing parry or dodge system from any other action game and tanks would have all the gameplay nuance already designed for them.
I'm not saying Blizzard needs to do any of this because that's not the kind of game that woW is, but if we're just going to say "It won't work" without trying to tackle the challenges, aren't we giving up far too quickly? I'd love for some action MMO to release that still has the holy trinity because it would probably be quite innovative and revolutionary.