If enough people that they want to follow give them the "mastodon.social treatment" as I'm calling it (which basically means rejecting them based on being on an instance with a bad reputation) they might, they also might not. I know plenty of people who choose to stay on Mastodon.social even though they're limited by lots of servers and users for spam. Same would probably happen with threads. Though I imagine there will be significantly more animosity towards threads than mastodon.social because threads is not just a bot infested mastodon instance, it's run by Facebook.