But you're giving Meta the same selling point, right? Join Threads and see all the same content. There's no point in going elsewhere then. It kinda goes both ways.
You're right that we don't know what will happen. So it could just as well be that Threads would swallow the whole Fediverse and then if Threads blocks an instance, it's like a death sentence for that instance. That's the whole embrace, extend, extinguish.