How could nostr use #pubky decentralized DNS?

Easy. Instread of connecting to the relay URL which is governed by DNS.

You can connect directly to the relay pubkey. If for some reason the relay DNS is taken down. It can move to another site, or a hidden service. Users are automatically updated.

To prevent this you would have to take down 20 million nodes all over the world (much harder than silencing a relay). And even then, 100s can pop up in minutes, using existing software installed on 100+ million computers.

That's just one use case. #pubky has 100s.