Agreed, cheap labor isn't hard to find, and there are countless people that would answer captchas all day long for a few dollars. How many captchas could you answer in a day? Let's say you work 8 hours, and answer one every 20 seconds. That's 1,440 captchas answered for $3/day. You get 5 captchas for a penny. If you can make more than a penny off 5 captchas and whatever you're trying to post or accomplish, you have a business.
It's already happening.
It turns out humans come quite cheaply, and you can actually solve captchas using humans at very high QPS for very low cost. And people do!
Site owners will have to come up with alternate means of "bot" enforcement that does not rely on human vs computer detection.