The idea is that people who care most about keeping it up will pin it, or pay to keep it pinned.
Perhaps it is also possible to tweak the caching algorithm to keep track of files that are rare on the network.
... and it also sounds like usenet, but here everybody is a news server and they selectively host articles.
I wonder if this would also be possible in this case?
For example, my small server might serve 200 sites but I'll backup around 500 or so older ones somewhere else.