I think there were other more critical reasons why hashcash wasn't adopted.
Yes, there is spam on nostr now. I'm saying that if POW would be required, the amount of spam on nostr would only decrease, not increase as some have said.
You may see inscriptions as spam on the bitcoin blockchain, but they are not spam (worthless) to the people who pay substantial material costs to create them. Spam has a cost, and raising the cost via POW (or mempool fees) will reduce the amount of spam that people are capable of producing.