You're not going to argue them out of Twitter. If they're still there, it's not for rational reasons. It's because of nostalgia, or because they're part of communities that are stuck facing the Fiddler on the Roof problem.

Shaming them for staying isn't going to work, either. We need to make a space for them to move to, not away from, and, frankly, the Fediverse just isn't that right now.