Yeah I think you need a centralized system with decentralized ownership, so that no single party can fuck it up by themselves

Just to be clear, what I'm referring to here is that a search would occur on a single instance. E.g. searches on lemmy.world occur on the lemmy.world instance, and load lemmy.world's servers. The federated part is in the building the database on lemmy.world. E.g. a crawler or a user on lemmy.ca adds a new web site and that record is federated to lemmy.world to add to its database. Another user on feddit.de upvotes a search result and that upvote is federated to lemmy.world so that the search result shows higher for users searching on lemmy.world. In this kind of model individual search instances could in fact be very large based on their usage. If there's no limit to what's federated, that would put a lower bound on the size of instances. If there's a limit (something dumb like federate only search records for *.fr domains) then that would allow for smaller instances that don't have the compute and storage for the complete index.