Proton has been gradually closing down access to proprietary apps only. After they’re done you won’t be able to take your email anywhere else.

If you have your own domain you’ll be able to host it elsewhere but you would leave behind email, calendar, aliases etc. and restarting from scratch.

At that point “encrypted” starts smelling more like “hostage”. It’s generally a bad idea to be tied down to a specific email provider.

You could wake up tomorrow to find out Proton has been acquired and the new owners can charge anything yet want for continued service.