If you can specify "I've got 1gb spare to replicate other people's assets, as long as each of them replicates mine in return" then your incentive to replicate other people's sites is more redundancy and bandwidth for your own.

If you also prioritise files that have the least replicas across the entire network, and allow servers to re-distribute the content they are replicating, it would surely become quite hard for a file to be lost forever.

This makes more sense to me than having each site pay for their own data to be replicated, because those who don't pay don't get their content replicated, which results in missing content when the server eventually disappears. That is just as bad as HTTP.