Certainly an aspect of my willingness to switch was that email encryption end to end only worked with other protonmail users, and unencrypted emails in transit

Certainly an aspect of my willingness to switch was that email encryption end to end only worked with other protonmail users, and unencrypted emails in transit are already readable, and they could surely be compelled to capture my decryption password the next time I used it anyway. They're still subject to swiss law which isn't quite as libertarian as protonmail sometimes implies. I think that for most people (or at least me) the attacks that protonmail is good for aren't especially likely and can trivially be rendered useless by the kind of adversaries they would theoretically have worked against. Disclaimer: not a security expert, just based on my impressions. Sorry if I got any details wrong. At least fastmail isn't Gmail sending your info to marketers.