People are staying on YouTube and enduring ads right now… Plus if Threads is federated they will be a part of the fedi, so no more reason to change. Why change to another instance, if you get on Threads without doing nothing and already are on the biggest instance ?
For the growing possibilities check what happened to XMPP. Is XMPP growing better now ? I don’t think so…
People won’t leave YouTube because content creators don’t want them to. That’s it.