It's interesting that you are talking about "two-tiered search would decentralize efforts to improve algorithms". But you mentioned that "this solution does not need to be distributed: sites can share their local indexes and ranking algorithms with the routing search engine."
Can you elaborate a little bit about why it's not a distributed solution if sites share their local indexes with the routing search engine?
Can you elaborate a little bit about why it's not a distributed solution if sites share their local indexes with the routing search engine?