Yes he could charge all users a subscription. But he can also have significantly less then 550m users. At the same time giving the remaining users less motivation to want to fund the service. Also advertisers less reason to use it. Leading to higher subscription costs. Leading to less users.

A bit like the mythic Oozlum bird. As musk spins around in ever decreasing circles. Untill he disappears up his own arsehole.

Hold on......

Hey Elon sounds like a good plan to me.