If you own it sure. However, if you’re suddenly changing contracts after the fact that’s just theft.

What if a backblaze or other backup service just decided it the copyright on all uploaded files after 1 month of non payment and started selling people’s home movies as stock footage?

Ah I see. It's a rules thing. In that case it's fine. The ToS explicitly allow this after all. So you aren't really changing things outside of what's allowed.

This is like when you put into a contract "Party A may withdraw from this contract at any point" and then you withdraw at some point. That's playing by the rules.

You can put anything in a ToS, that’s simply not enough. Again, I would be fine if Google simply stopped hosting videos without consent, but behavior changes like this are different.

How is it different?

If you want to make changes to your assets after you die, write a will and hire an executor.

The living aren't obligated to negotiate with the dead.

The issue is you can’t guess every possible change. Let’s suppose Facebook goes broke in 20 years and the new owners decide to make absolutely everything public. That’s going to make a lot of people upset whatever it’s allowed by the TOS so that’s fine right?

If your alive you fight such things by suing the company, but the dead don’t make such choices.

For better or for worse, dead people (or rather, their estates) can still have copyright ownership of works they've produced.