You miss a big difference between cryptocurrency and PoW captchas: it's very hard to change hashing/PoW method in a blockchain (you need ack from 50% + 1 validators, and all the validators are by definition heavily invested into the current proofing method - they have natural reason to object), but it's very easy to change hashing in a PoW captcha. So here any cheating will start a whack-a-mole game, where the defenders have the upper hand (changing PoW method is cheap, using multiple PoW methods is cheap, investing into ASICs isn't).