There are off-chain solutions to handle most of the payment, and only put a summary on-chain. I think there are already micropayments in Brave or something.
Proof-of-work crypto is interesting here because it is fungible with computation, so these solutions that charge computation to users are literally equivalent to crypto.
It's a solution that already has adoption, does not require everyone to sign up with a centralized service, and does not require everyone to pay money (they can pay with small amounts of computation instead) so it remains accessible to ~everyone.
Yes, sites could use a bot protection service that runs captcha breaking AIs on the viewer's browser. Said bot protection service could then break captchas for forum spammers to make real money.
