Yes and you can't run a bridge on anything

You can run the bridge on Windows, Linux and macOS. I wish that they had an Android app but I suspect that the aggressive power management on Android might make it a lot harder to implement in a nice way.

they might discontinue it at any time and you'll become hostage.

There is absolutely no indication that that will happen. Remember that only the paid plans have IMAP and Proton is essentially held hostage by their subscribers since they are their only income source. Their community would be very angry if they dropped support.

So much for self-hosting, independence and open-source solutions.

This has nothing to do with self-hosting. You are literally paying someone for hosting your email.

I agree that the lack of normal IMAP support is annoying. But it's a side effect to their encryption which is a good thing.

And it's not like proton is alone with this problem.

Tota doesn't even seem to have a bridge app. Fastmail changes extra for IMAP.