While I agree that it is probably better to not defederate them right from the start, I believe that the Fediverse might not have that much leverage. This recent blog post about the history of XMPP describes it pretty well: https://ploum.net/2023-06-23-how-to-kill-decentralised-networks.html

As expected, no Google user bated an eye. In fact, none of them realised. At worst, some of their contacts became offline. That was all. But for the XMPP federation, it was like the majority of users suddenly disappeared. Even XMPP die hard fanatics, like your servitor, had to create Google accounts to keep contact with friends. Remember: for them, we were simply offline. It was our fault.

Of course it's different because this is social media and not just 1:1 privat messages. But for us to actually have some leverage/impact we have to generate a lot of (good) content so that Threads users will actually notice and complain if that content "vanishes". And at the same time we must not become too dependent on their content