Indeed. I think the only thing close to WoW in ¡14 years! (man, I have characters older than some players) is FF14. It's a lot of time to some game "steal" their niche.
Talking of niches: that's another factor. If you're invested in one MMO, it's very unusual that you change to another one, unless the original one fails to deliver your expectations.
The problem is that investors direct videogame companies like if they were movie studios: if there's some hit, everyone and his dog follows up with copies of the original hit. Some have luck, but in general is an assured fail, because while a movie can be seen and be done in 2-3h top, a game demands much more commitment (and if it's an on-line game much, much more).
That's what happened with click & point adventures, shooters, MOBAs, survivals, etc. and what we're seeing right now with battle-royales.