I think Apple solves this pretty well in iOS… there’s pretty simple iCloud API’s apps can use to store basic data (also structured data with CKDatabase/etc) which automatically just use the user’s iCloud account storage and implicitly are synced across devices. Apps don’t have to implement any cloud storage or consensus/etc, they just use the platform API’s and everything just works. I’d be shocked if android/play store didn’t have a similar thing.