Why can't syncing data be commodified via a dumb cloud that just syncs blobs? If you want privacy locally encrypt everything.
We already have that in the form of S3 and friends, but they aren't consumer-accessible. We also have some platform-specific ones like Apple iCloud but those are platform-specific.
In the end the problem is that there's no financial motive to do this.
(BTW, your idea supposes the ongoing existence of storage services with a common, stable API an app can be written against. You can barely say that exists now, much less expect it to into the indefinite future. The previous poster only assumes general servers are available, including BYO if you want.)
I plan to offer single click resale of storage directly in the app for users who don't want to deal with access keys, secret keys, ACLs, and the like.
So even if the software and all supporting software is no longer available you can still make something to read the data..
This is kind of getting into r/Preppers territory, but still... There's something nice about that.