i understand what you mean with oldschool MMOs and i long wondered why there hasnt been a CRPG that emulates oldshcool MMO raiding with RTWP combat.
That beeing said, DnD as a ruleset doesnt lend itself to that. Maybe 5E does because it gets very high HP numbers on higher levels. But class mechanics in 5E arent very differentiated, thats because DnD is a game where an idividual plays a class, so its less important for a Sorceror and a Mage to have different Spells as it is to have different mechanics of attaining them.
Well, that appears to be the current philosophy anyway