I suspect that many of the ones that came in the most recent WoW exodus (it was not the first and won't be the last) didn't stay because they didn't really come to play FFXIV. They came to be part of the band wagon following their favorite streamer. When that streamer bailed and went back to WoW (a likely thing to happen since the streamers career was made on WoW) the followers bailed as well.
Even so there is always the potential for a few of the escapees to decide to stay and some did.
I am the product of 2 different WoW escapee waves (neither because of a streamer in my case). The first was right after the release of ARR when I was fed up with things in WoW and searching for another game. I created a trial account and character and got massively confused by the game within the first 2 days. I had no one to help and the one person I found to "help" dropped me in a zone way to high for my level 3 self and vanished. I had no idea how to get back to where I needed to be so I just logged out and gave up on the game.
My second try happened when I was well and truly done with WoW mid BFA. I remembered I had that old trial somewhere for FFXIV and decided to give the game another try. I managed to revive the trial (Surprisingly it revived as a trial). I couldn't remember what server I had been on so I made a new character (the one I am posting on) and started over again. Something just clicked this time. I fell in love with the game and haven't looked back since. I have since gone back to my original character as a main and rarely play Myrany anymore.
For those interested my initial problem with the game as it turns out was a lack of knowledge of the transportation systems and how the map works. Not only could I not find where I needed to go I couldn't figure out how to get there.
Some refugees do stay and do integrate into the community just fine. Not all will stay. Some like me will eventually come back and try again.