> There's no asymmetry at all: you're just adding a fixed cost to each reward, for both users and abusers.
There is: users solve the PoW once, scrappers solve it every time they rotate IP addresses. Some of them change their IP address on every request, so they keep solving the PoW over and over.
That doesn't matter! Every time they successfully solve the challenge, they're getting the token reward. That's the missing asymmetry. In an anti-spam system, 99.999% of all transactions are unrewarded.
There is: users solve the PoW once, scrappers solve it every time they rotate IP addresses. Some of them change their IP address on every request, so they keep solving the PoW over and over.