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

I guess this is when it gets fuzzy. A true peer-to-peer social network would mean that every node has to be able to get everything and also be able to share whatever it has with every other node. If every participant on this kind of social network was required to maintain their own "seedbox", then it implicitly limits access to only people with means.

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.

That's more or less what atproto is, except the indexer is called a relay and provides the data directly. Currently bluesky is providing their relay for free, but I don't believe there's any reason they couldn't make it paid in the future.