Then they will simply solve the captchas using captcha solvers or humans paid pennies per captcha. You do something else and the botmasters will respond.
It's really hard to get rid of bots and fake accounts. Also, false positives are expensive because you end up annoying a real human and as a result your mistake doesn't disappear until you make it right.
The cat and mouse game of bots vs captchas has been going on for a long time and has become increasingly sophisticated. "Just use a captcha image" is a 10 year old solution, if not more.
How would they know to write that code if they were not expecting it? Just do it as a one-off on a particular day without any warning. And you can show it to some small sample % of total users then extrapolate, so that the average botmaster wouldn’t even notice.
