if threads get hundred million people in a short period and they start to write and interact with each other then that a good moat for them.
Might be that BlueSky is not trying to be a popular social media platform. But if they want to then having right now their signup closed is a very bad decision.
It is already hard to keep track of where each discussion is: twitter, mastodon, instagram, facebook, linkedin ...
I think people are maybe trying one more from what they currently have.
look at Clubhouse - they tried the invitation idea and died with it in their hands. When having so many platforms available almost no one waits for yet another one for 3 months to get their handle to finally talk with 10%of their existing network as their other 90% still waits their invitation.