Mastodon is too convoluted for the average person to be a true alternative. Haven't checked out Blue Sky yet and quite frankly don't really care too. Threads will be around whether people like it or not and I think it will be theplatform that we see celebrities and brands prefer. It's just about making sure they listen to what the users want out of the platform and hoping they implement features.
As someone pointed out, having Threads connected to Instagram was a brilliant move. IG has over 1.5B active users/accounts and every single one of those can be turned into a Threads account since they made it super easy to set up you Threads account though Instagram in only a few steps.
Zuck has also laid out his approach to monetizing Threads. Essentially, get on a path to 1B active users before ads.