"But why would any dev start to do such a thing in the first place?"

A hypothetical scenario: Because they believe that anyone who disagrees with their political beliefs is a Nazi and should be silenced

"Why would users addopt it secondly?"

Because they are the best clients technically, and these users do not really care about politics. They are just normal people who want to use social media.

Yes, you can fork the clients, but non-political people don't care enough to switch to your fork.

I'm not saying these things are happening right now. I'm just saying that a scenario similar to this is possible. There is nothing in the nostr protocol to prevent this. I really hope I'm wrong.