All that happens if a spammer validates pubkeys on their domain is that their entire domain gets permanently blocked.  How fast can they spin up new domains?  Not very.  We win.  NIP-05 is still useful for spam prevention. 

 🎯 

NIP-05 is not perfect, but it is very useful, as you correctly point out.

No single spam prevention tool is ever going to be perfect. Luckily, LOTS of tools exist. Let’s learn to use them all. 

Abundance mentality.