> every instance operator can make their own decisions on this matter.

Not entirely. "The fediverse" has this concept of "fediblocking". If a large enough group of admins decides they are very much against this, they will not only block Threads, but block anyone who does not block Threads.

Basically strong-arming every admin who does not follow suit, to do as "the hivemind wants it to".

This has some positive sides to it, but it's also a weak point in the idea of decentralization.