Humans are the problem, not some mythical AI powering the bots. It costs a fraction of a cent to get a CAPTCHA solved. The only real counter being used is making it take longer to solve a CAPTCHA - which is exactly what the services like reCAPTCHA do, while minimizing the impact on heuristically 'good' users.
Making CAPTCHA solves take 30 seconds or a minute instead of 5 seconds is the state of the art.