I think users are already too reliant on servers, and the last thing we would need is yet another commercial server hoarding a lifetime of data & metadata.
I would enjoy seeing the mentioned reciprocal scheme for peer to peer message backup based on secret sharing. However, note that even if such a system were to be implemented, instead of Delta Chat implementing it separately, it would be desirable for all apps on your device to use it, but then it would need to be running a daemon for each and keep syncing with subsets of peers for each. This would be quite inefficient.
Here are answers to some of your questions: