A service equivalent to Twitter does not cost billions per year to run. In principle, you could boot one up with pocket change and operate it with a small budget. The reason you can't do this in practice is that the network effects are a fundamental requirement, almost impossible to recreate, and all the established operators are forced by shareholder duty to play the adtech game.
If you were an effective altruist worried about adtech's pervasive influence on society and had unlimited funds, you could hardly do better than purchasing one of the few established, global social networks. Trim down its cost structure to the bare minimum, make the economics work without letting the global-reach advertisers dictate terms, preserve the network effects that comprise the real value of your media channel and then shape the discourse to serve whatever positive outcome the current system prevents. You'll have the only privately-owned, global-reach media channel in the whole world that's not fundamentally influenced by advertisers.
Of course, Elon Musk might not be an effective altruist and the billion-dollar interest payment is an impediment to the above plan, I'll grant you that. But hell, I wouldn't be surprised if the guy just ends up subsidising this thing from his other businesses if he can't make it run organically. That's what I would do. It'd be a uniquely independent and influential media channel.
I really don't see why this would make people quit Twitter en masse. The only people quitting Twitter are those who really hate Elon Musk, and there just aren't that many of them. The service works like before and it might well work better, allowing democratic discourse that is viable for a global audience nowhere else.
As I mentioned earlier, the conclusion boils down to whether you think Musk means well or not.