I don't really watch many streamers at all, but a lot of them simply won't survive without playing WoW. The ones that will survive are the really big ones that are actually charismatic and don't have boring streams. Whether you like some of the big WoW streamers that jumped over to FF or not, it's hard to argue that they don't have a following or are watched for very particular reasons.
One of the biggest fears of streamers (as somebody who dates one who have a several hundred followers) is actually trying out different games. A lot of the time people watch you specifically for that game and more often than not, your viewer ship doesn't translate over to new games. A portion who likes you for you will watch you play whatever, but most people are hyper fixated on one game at a time, and unless they decide to move as well, you aren't going to get those views.
Everything that's happened to Blizzard in the past few months, let alone last few expansions is basically their own fault, and I say this as somebody who still mostly plays WoW as my MMO. The game hasn't evolved since Legion (this isn't me even saying Legion was amazing BTW), they had a huge delay in .1 content, scandals, and like I always say in any thread discussing WoW, it's a game that's been out for coming up on two decades. Peoples attention no matter how hard you attempt to keep them playing your game is going to fucking wane and that grows with each and every year. WoW has used pretty much the same formula for going on 5 years now and the game has the perception that the developers don't really listen to feed back.
It's a lot of monkey see and monkey do though. When a lot of WoW players trying out FF for the first time eventually reach end game with very little to do (this isn't a bad thing BTW), what are they going to do? There isn't infinite content at max level, and a lot of the max level content in FF basically is dressing up your character.
Negativity sells BTW and I'd be concerned if I was a primary WoW focused streamer with a few hundred viewers to even a thousand or so, upping and quitting WoW. A lot of them aren't nearly as interesting and their viewership will massively tank, just like when streamers jump to other games because it's the "thing to do". If you do high end M+, raid at a high level, or god forbid do anything related to arena you're pretty fucked if you try to jump ship. You see trends with games all the time and it's not just with games. People meme about what the 'hot girl meta' is on a twitch, and it's the same with games too. Suddenly everybody is playing Fall Guys, followed by Among Us, and then you have games like RUST popping up from time to time. FF14 isn't really that different, but I doubt it dies down anytime soon because it's an 8 year old game with lots of content to chew through.
You generally get views from being highly informative, really good at the game you play, or incredibly charismatic. Most WoW streamers aren't number three on the list, and trying to learn a new game, let alone become good at it in a quick amount of time (assuming you even pick the next big thing) is a huge risk.