I feel like this has been being said repeatedly (remember the "Twitter will irrecoverably go down any day this week!" craze from late last year after the layoffs?) and yet hasn't really materialized in a meaningful sense.

Sure, the fediverse has been growing, but most users haven't left Twitter, they've just also made an account on the fediverse. I don't see how this will be any different. It's already normal for people to have both a Twitter and an Instagram account, or even stuff like both a TikTok account and posting shorts on YouTube.