You are taking this massive “if” and building a whole policy of preemptive panic around it:
I don't see how it's a "massive 'if'." If it was just some fringe possibility, I wouldn't be so concerned, but the thing is that I don't see any realistic scenario where we don't become dependent on Meta for microblog activity. If 99% of microblogs come from Threads, that's exactly what's happening. To give an example that's more relevant to the thread aggregation side of Kbin, if Reddit were to federate and we didn't defederate, Reddit would make up 99% of the thread activity we see, we'd get used to that, and we'd be completely dependent on them to maintain that. With how desperate people seem to be for a quick boost in activity that they'll just take whatever Mark Zuckerberg offers as if there are no strings attached, I don't see how we just end up fine if Threads is to ever leave in the future. If Threads becomes most of what we see, we'll be dependent on them, and if Threads then leaves (which they have incentive to do), much of who we have right now on these platforms will join Threads after getting used to the activity, and getting new users will be much more difficult.
And if we don’t and we defederate, we’ve just cut off potentially interesting conversations with interesting people based on ideology.
That's definitely true. Again, What Meta is essentially offering is free activity on a silver platter. What's completely nonsensical is to act like there aren't any strings attached when there are obviously strings attached. Meta is trying to maximize profit. Anyone who thinks that Zuckerberg suddenly cares about an open fediverse even though its values (people being on multiple instances, everything being transparent, no one person or group having too much control, etc.) go directly against his goal is either delusional or very misinformed about what these for-profit tech companies do. It strongly benefits him to take users from Mastodon, Firefish, Misskey, Kbin, etc., and allowing ourselves to depend on him for fediverse activity puts him in a prime position to do it.