After all, if enough of them do it, and they all get banned, the platform's dead.

Possibly, but they're narcissistic, dopamine addicts - they need their social media fix. They'll throw a tantrum, but I would imagine they'll come back as the platform is too familiar, too much part of their lives to let go.

I struggle to imagine normies ever using Mastodon, but it'll be interesting.

That too.