IDK, I think that's kind of flawed logic. E.g. "we should stop using gasoline for cars and switch to electric" - would you say "what a stupid thing to say. everyone uses gas so we shouldn't try to stop"?

And are you not aware of Signal? It's open source, and the default server is not, but it doesn't matter since it is E2E encrypted, just like Proton. The difference is that ProtonMail allows you to communicate unencrypted with non-ProtonMail accounts.

I think all they're saying is that, similar to gas users, there are many people who will not stop using it or just don't care unless we sunset gas cars / email for them.

I do agree with that, and in both cases it isn't something which can happen overnight, but it is a serious long term problem which IMO we should be pushing to solve.