But you don't own it. X does. It's their service, they are free to apportion handles as they see fit. It is nothing like a house where you have an actual ownership claim through the deed.
But it's your identity. It's the way you've been putting yourself into the world and telling people they can reach you there. It used to be that if someone sent a message to that address, or tried to navigate to that address, they would reach you; but now, they'll be taken to somewhere else, and they perhaps won't even realize what's happened.
And for the ownership issue, sheesh. Yes X, in a literal sense, owns all the usernames. We're talking about whether it's morally right for them to do, not about whether it's illegal. If they had held back these short "valuable" usernames from the beginning, no one would care; it's the act of taking away someone's established identity that is problematic.
It wouldn't have been so successful if everybody be called "Anonymous" meaning that they wouldn't be able to make money with it.
They've started to take this away now. Today it's some account with obviously few words. Tomorrow it might be one with wrong words. What you counted as value is nothing. It might be lost tomorrow, so why bother?