And in fact people who who want to interact with the 140million ish Threads users currently have one option - join Threads. With federation I can communicate with Threads users without joining Threads.
What if the defederation happens in the other direction? Defederating an instance is a lot like banning a user, and I'm not sure if there are any mainstream social media sites that I haven't heard abuse their ban system. If other instances start becoming more popular because people want to use them to talk to Threads, that gives Threads a lot of power over which of those instances are allowed to thrive. In the worst case scenario, it could easily kill an instance if too many of their users were there for Threads and Threads decides to cut them off.
A fediverse that is popular because it can talk to a centralized app doesn't sound like a particularly healthy fediverse to me.