Comment by bobbiechen

Yeah, I had the same question myself. I think that's what you would want to do to make it airtight (plus some amount of rate limiting or flagging for devices that are part of dedicated device farms).

But even if not, there's still value in raising the barrier to entry. For example, you can buy 1000 reCaptcha solves for $1-2 from various captcha-solver services. And yet that $0.001-per-request fee does discourage mass-scale bot attacks.