This probably isn't so true for the pay email services, but I just wanted to note that these two things are by no means mutually exclusive. Plenty of services are more than happy to take your money and "make you the product." It's so easy to do on the web, few can resist.

For me, I confess paying for it has the risk of it being held hostage if I miss payments. Worse, my data can turn into leverage for increases price.

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.

I'm assuming bitrot of your backups is also a concern. Or do you have a suggestion on automating integrity checks of your data?

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