On a separate but related note - I think we can expand NIP-50 to offer a lot more optional functions in the search, including one you mentioned.
Isolating results to only one relay (even if NIP-50 relay is an aggregator), negative keywords, wildcards, topics, language, sentiment, nsfw, etc etc.
Obviously I don’t expect every relay to implement these, but even if we only have a handful of aggregators we can query directly, thats at least on the decentralization spectrum compared to all existing social media search. We should also expect the number of aggregators and search providers to grow with time if nostr succeeds (even if their data become more fragmented).