I agree paying a relay solves the problem of spam for that relay, but that's not necessarely applicable to every service. Some services aren't directly monetizable (not because it's impossible, but because people won't pay for that), and thus another spam prevention mechanism must be used.

Also, many people have PoW keys (you included), so there is clearly demand for these things. My intuition is to use these and/or PoW endorsements for anti-impersonation and spam prevention.

> unless there is a difficulty consensus you are fucked, whoever has the most compute at their disposal can drown your "spam protected" channels.

yes, it can happen, but:

a) that's not going to kill nostr, unlike these shitcoins you mention

b) it's going to cost the attacker a lot of money, and the attack will be public, so people will stop using this spam prevention mechanism and the attacker would have accomplished very little