Eh, Syncthing is only stuff you control, which doesn't exactly fulfill what OP is talking about: extending the benefits of self-hosting to those who can't or don't want to self-host. It also doesn't expand storage, it just keeps your storage in sync between devices.

P2P solves a lot of this. It provides expanded storage, can be easy to get into (add nodes as you go/pay others for nodes), etc. But there's the perennial issue w/ trusting others w/ your data.

That's why I think a hybrid is better. Buy storage from trusted providers as needed and use apps that work w/ that. Unfortunately, that doesn't seem to really be a thing, but I think it could be super cool. Places like libraries can provide libraries to underprivileged people, who can then add to it w/ something from the market.