I've seen estimates that 65M Americans have twitter. 65M/330M is definitely enough of a critical mass to start something big.
Effectively, Musk paid $675 per (American) user to bootstrap his everything app. Probably easier to incrementally add features to the-app-formerly-known-as-Twitter than to build something brand new, even if you can get a ton of sign ups when it is first announced. Take threads as an example, people got excited for approximately 1 minute, then forgot about it.
Google has probably had 100% of every American adult and child in the past 20 years, and they haven't made a dent in payments, even with their own phone ecosystem. This 65m/330m back of the envelope stuff is pure fantasy.
Unless people have no option but to use it, it won't work. For example, the onl reason I use Paypal is that 20 years ago, Ebay didn't offer an alternative. Today, I use it because I can unsubscribe from stuff with one click. I'd never use it otherwise.
Threads died because the content trends was consistent with that of Facebook, highly monetized and therefore unappealing to Twitter users. Totally irrespective of Threads, Facebook is a superior replacement to Twitter in terms of its systems, and the value of Twitter is its violent flow of unmonetizable content generated by explicitly anonymous users, and so the whole thing is where capitalism goes to die and financially there's just no way out.
That billionaire man paying generous amount to users would have been a fortunate event to users collectively, but that's it.
Effectively, Musk paid $675 per (American) user to bootstrap his everything app. Probably easier to incrementally add features to the-app-formerly-known-as-Twitter than to build something brand new, even if you can get a ton of sign ups when it is first announced. Take threads as an example, people got excited for approximately 1 minute, then forgot about it.