The whole argument is that Meta will do whatever they want with their implementation of Activity Pub and lacks any further details. Blast radius of what? How does that affect existing Mastodon instances? Do they lose anything compared to what they have now?
Threads doesn’t need Mastodon users because it has orders of magnitude more already. Mastodon has unique competitive advantage, for example no ads, that could compel Threads users to switch with little friction. It might turn out that Threads will offer things Mastodon won’t on principle (follower and notification management for huge accounts) which might actually make whole ecosystem more healthy and diverse.
Really, it’s best to see what’s going to happen. I’m optimistic because I think open alternatives are generally better and will win long term.