The fact that "yeah but a significant number of users always could just leave" is always out there but I'm not sure we have much info on how, when, why, how likely it is for these things to occur.
I also wonder how much celeb impact there is, if any, vs just volume of acquaintances. I suspect different networks are different in that regard.
In the Facebook era this changed for a number of reasons, not least that Facebook attracted a "normcore" audience of family, friends from school and other people who just weren't interested in being part of the latest scene.
Certainly since the X-odus started we've heard from many online celebrities that they don't feel they can move because they are afraid that they won't be able to rebuild their audience. I've been talking with a friend about this problem (and related problems) and personally I think Bluesky's got it. Bluesky is doing a lot visibility and behind the scenes such that left wing journalists with 95k followers on Twitter can rapidly jump to Bluesky and get 250k followers -- and higher quality followers because most of these people just joined and haven't had years to abandon their account. Each time somebody like Springsteen moves over other celebrities will see they can do the same.
For journalists in particular, I suspect that that it largely because Twitter appears to heavily suppress posts with links in them; a "hey I wrote a thing in [newspaper]" post on Bluesky may end up with a lot of visibility if people are interested in the article, but on Twitter it will have practically none, because of the link thing.