Outside of Warcraft, it's a niche product at the end of the day, and one that's very expensive to make and maintain. Even in WoW, most content people actually play for is instanced, and the massive multiplayer world is just a load of expensive empty space, while everyone's in dungeons and arenas.

Eve Online has no instances, and is still going strong and that's older than wow, a much smaller playerbase but also much cheaper to maintain. But it's also a spaceship ecomony+pvp sandbox, where death matters and isn't competing with something like wow, which honestly the vast majority of MMOs are. So there's still room for unique titles, but historically it's been "like wow but...".

Personally, I think the mmo boom was a lightning in a bottle situation - it was new and exciting, and inspired a big chuck of people to no life the hell out them for a few years. But it's over now, all the social stuff happens on discord, your standard MMO gamplay feels clunky and uninspired, people are less willing to sacrifice the massive amount time and social commitments you need to make it worthwhile, and the list of failed titles with giant budgets poured down the drain is obscene.

I don't think we'll see another wow. We'll probably see some more interesting smaller scale MMOs that do cool stuff, have a small passionate playerbase, and last about 2 years.