I can't speak for anyone but myself.

I am not attached to the fediverse. The federated aspect is, to me, interesting from a technical standpoint, but irrelevant to my decision to be here. I'm also not particularly attached to foss principles.

I came here because I got annoyed at reddit. I'll continue to poke around here exactly as long as it's entertaining/informative. That purpose is not contradicted by leaving here for threads (it'd have to be a reddit clone instead of a Twitter clone to pique my interest at all, but leaving that aside).

So if, over the next few years, more and more of the content that I was interacting with was coming from threads, then threads split off, it's reasonably likely that I'd want to continue interacting with threads. And if the majority of the stuff I was interested in was on threads, I probably wouldn't bother coming back here.

A reasonable reaction to that is "don't let the door hit you on the way out", and that's fine. But what could conceivably happen is that something like threads uses what has been built here to gain ground, then starts leeching away communities. They start moving to Facebook servers because Facebook has butt tons of money so the servers are stable, and besides, everyone else can still get there from other instances. Then Facebook starts adding incompatible features, which motivates more migration to their instances, and so on and so on, then there's a split.

Now Facebook's threads has devoured your communities, taken your users, and so taken your content, mostly just to jumpstart it's own growth. To get what they are familiar with, people like myself stay on/move to Facebook, leaving the fediverse to rebuild the communities that it built in the first place, out of the people who care more about foss principles. While appearing to external observers like an inferior clone off the Facebook threads thing, to add insult to injury.

So the issue is that you wouldn't lose just the new stuff from Facebook, but a fair bit of the preexisting stuff that sided with Facebook after the split out of convenience. What you'd keep are the people who stick it out out of loyalty to foss or federated or some other principle - and that may not be enough to carry on the level of content that's desired, even with the fediverse's "size isn't everything" philosophy.

Of course, it's possible that either a) none of that would happen even if there were federation with Facebook and everything would be fine forever, or b) all of that would happen even without federation with Facebook, just without the intermediate stage where there's interaction. But the above seems to be the concern, and it's not without merit (both because of past examples and, well, because I know I personally wouldn't stay if the content were more appealing somewhere else and I don't think I'm unique).

Personally, I think that with Twitter reeling at the moment, all the Facebook version has to do is be similar enough to be familiar, have good performance, and be easy to use to have a shot at that nabbing that part of the market (including users from the fediverse) - interoperability with mastodon or not. But predicting the future is rather difficult, so it's hard to say.