On the other hand, twitter is a pretty global platform, yes a big share of its users is from the US, but japan, brazil, the middle east constitute a huge part of its userbase, so while I wouldn't say that losing the part of twitter that hates elon and his policies won't be a problem (if that ever happens), it's impact won't be as large as tumblr, because no one cares about elon in japan.
The topics discussed on twitter are also far less niche and more current, politicians, celebrities, artists, companies all use it, so it mostly can't experience meltdown by changing its policies, because what's keeping people on twitter is other people, not the policies. Yes this does mean that if those people leave every one will with them, but there is simply no alternative to twitter to leave to right now, mastadon is a joke, discord and reddit are just a different form of social media that wouldn't satisfy twitter users Maybe someone can make a new platform just like twitter? but I doubt that elon's decisions can be that bad.
He mostly has to keep it the same, and he won't lose a single user.