The thing is, a lot of people own more than one device. Even more people know a couple of people who have devices with some spare space. Wouldn't it be great

The thing is, a lot of people own more than one device. Even more people know a couple of people who have devices with some spare space. Wouldn't it be great if we had a way that people could "join" trusted devices to create mutual background backups. So I could pair my laptop with my phone, and each would back up "essential" data for the other. But I could also pair my phone with my significant other, and our phones would also work as backups of each other. Syncing could be done on some kind of schedule, maybe when they're in bluetooth range, or maybe if they find each other on the same LAN. Or maybe they could use cloud storage to transfer E2E encrypted blobs. (i.e. my phone would backup my laptop and my parter's phone, my partner's phone would back up my phone, and my laptop would also back up my phone. If my partner wanted/was willing to also back up my laptop, that would be a separate step, and they would back up each other.) Deciding which data should be considered "essential" is left as an exercise for the implementor :-)