Eh, the reason DNS is a problem is because there is a central source of identity that can be tampered with. If relays just used npubs to communicate their current IP, there isn't a central point of control. Swapping out IP for something fancy might provide some additional defense, but at significant additional complexity that will have bugs and possibly back doors. We could continue to use IP with Tor or Whisper to get similar benefits using proven technologies. 

DNS is bad, but IP is mostly harmless