Yeah it's hard but fundamentally important. I think it's really difficult for such a system to be as valuable as closed source in terms of audit-ability (encryption is worthless if product is backdoored) and application development. I don't have the answer as to what would incentivize anyone to create such a system but it wouldn't be the first piece of important open source infrastructure that has been created.

Space Monkey looks really interesting but I agree that this isn't something that is going to perform well running on residential internet connections. It also doesn't look like there is anything like non-explicit sharing (i.e. public files) which I think is an essential part of a distributed data store because it would be a boon for small-time publishing (no more web servers!).