Lootbox gambling and instant gratification as gamedesign philosophy killed longterm commitment games, zoomers don't want to sink too much time in it without getting constant thrill that other games offer.
WoW being a monopoly pretty much killed every other mmo bar RS/EVE, people just don't want to play other games on empty servers.
And WoW killed itself as a mmo with LFD. Its not concept's problem, but rather whole game industry being in a shitter.
Also there is no strong IP to build mmo on nowdays so why even bother, especially when lootboxes sell better than games.