I think you will find that Meta will chuck N amount of money to make it happen with some of the Mastodon admins.
And if you don't like it, you can move to another smaller instance, however that said instance may not federate with Threads, otherwise setup your own (unlikely anyone other than techies will do this)
On every single threads.net profile, for example zuck.
"Soon, you'll be able to follow and interact with people on other fediverse platforms, such as Mastodon. They can also find people on Threads using full usernames, such as @zuck@threads.net."
mastodon.social and other instances have signed an NDA and have not denied federating with Threads.
And mastodon.social and other big Mastodon instances will allow it.