You don't ever know the specifics of the trajectory such as "selling to Elon", but the general arc under commercial pressures is predictable - that's the thesis of "enshitifcation".

I understand that not signing up in the first place is a valid choice.

But I also feel that using these services, on the understanding that they don't guarantee to remain good, and so you don't guarantee to remain there at all when they succumb, is also valid.

You don't owe them that. Another one will be along in a minute. That's why we went from MySpace to LiveJournal.

"legitimacy" is not binary nor eternal, and Elon's mouthpiece is leaking it's remaining legitimacy.