A lot of people aren't really ideologically opposed to Meta, they're just on Mastodon since it's there's less friction to use it than Twitter (see rise of Bluesky). Threads will "fix" a lot of issues people have with Mastodon (CW, no algorithm, inability to advertise, instances moving/going under) and they'll move without thinking anything of it since they can still access all of their Mastodon content

Of course the move back isn't going to be as easy, I doubt Meta is going to implement robust account migration, and then the easier choice is to stay on Threads. This is also ignoring the ~~incompatibilities~~ improvements to ActivityPub that Meta will introduce later on in Meta's lifespan, which will be poorly documented and rapidly changing if they open it up at all

Even if many Mastodon users don't switch immediately, this is enough to hamper the long term growth/health of the platform