This seems to confirm my critique of "manual" solutions with torrents and such offered in other comments, resulting in the idea shortly described in the comment you were answering.

Yes, this would require a lot of people, but some would contribute more and some less, just like with other public P2P solutions.

From my POV the biggest problem is synchronizing indexes (similar to superblock maybe) of such a storage, and balancing replication based on them, in a decentralized way. Because it would seem that those indexes by themselves would be not small.

There should also be all the usual stuff with controlling data integrity.

I think it's realistic to attract many volunteers, if the thing in question will also be the user client, similar to Freenet and torrents socially, and bigger storage will allow them to faster get things they access more often, as a cache. But then balancing between that and storing necessary, but unpopular parts of the space, is a question.

I think I need to read up.