how i would make it work is you'd only download actual bits you care about, the rest can be handled by indexers you pay for to help you sort it out
But if there were hubs funded by groups of interested people that allowed those folks to share the cost of "seeding" and in return only "leech" the information they care about... then isn't that essentially the same kind of decentralization that already exists on the fediverse? The way I understand it, instances are set up by communities of people with similar interests and those instances are configured to only propagate a subset of the events that their community is interested in.
anyways the idea is very much to do like bittorrent or kademilia but for social media posts i need to research more the fediverse before building anything tho