Copy pasted from my other comment:

There’s been lots of discussions about this already, but the short version is that this is a play book tactic used by big companies to take over a sector.

I didn’t experience this at the time, so the details might be off, but one example, as I understand it, is what Google did with Google Messages a decade ago or so. They federated using some other service that existed before Activity Pub I think, and most casual users just flocked to it with no idea about any federation. Once it got big, they just stopped federating, and anyone who was not on Google Messages but had friends who were (which most did since GM was so popular) had to change to GM.

Other similar such things have happened before with other products and in other markets, and that’s what people fear will happen with threads.