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.