Which is why you should always choose a service which offers full access via standards-based protocols so that you always have the option of owning your own data.

And why I regularly download backups of my email from Fastmail.

The only concern I have with them raising prices or disappearing entirely is the service interruption of moving my DNS to some other email provider.

Completely agreed. You should also not turn yourself into a hoarder of data just because you can. :)