But, users can never blacklist 0-day bad relays. It would be a never ending, constantly growing blacklist because there's no upfront costs to setting up a malicious relay. Unlike lightning nodes, that require an investment in Bitcoin to open channels that you put at risk if you attempt anything malicious.
Every day, users would have to put up with the shit from the latest spam relays pushed to their clients, as well as updating the blacklist of the spam relays discovered yesterday.
Spammers will flood any gossip protocol with hundreds of spam relays each pushing tens of thousands of bot generated spam notes.
That is no way I'd ever use a client that kept adding random relays discovered by a gossip protocol. It would also make Nip-65 useless because so many other accounts would be full of gossip discovered spam relays.
Relays should operate by reputation. If I offer a no NSFW, no politics, no crypto filtered relay that had a reputation for filtering everything except cat photos and cooking recipes, shouldn't people only ever use this relay by choice, and not have it forced upon them unknowingly by a gossip protocol?
Just as relays flooded with Alex Jones Protein Shakes spam should never be pushed to anyone.