Yes, backing up is the most obvious reason to have it live on a server from the user POV. And of course the app vendor will want more lockin and be able to keep the data hostage.

Paying for apps doesn't help as much as expected, as they'll want to keep the revenue stream alive. Solving this conundrum would require to deal with both side of the coin.

The real solution to this would be to give everyone and their dog a standardised online server with legally enforced privacy, and have sandboxed apps manage data in and out in a interexchangeable format, but I feel like I'm asking for peace on earth.

PS: we have banks for money, there should really be something similar for data.

> PS: we have banks for money, there should really be something similar for data.

It's very hard to imagine a government who would have this too far up their list of priorities. Switzerland maybe. Then Germany, France. Maybe. It's hard to imagine it catching on between the monied interests who can influence government and the lack of consumer awareness of the problem it is solving.

Not quite what you're suggesting, but at least Nextcloud (and other open self-hosting platform) servers are a thing.

All you need is for the app to provide the option of saving to local storage – your cloud server software provides a local storage endpoint and does the rest.