This is what hCaptcha is currently doing, they are switching the image category every 24-72 hours.
How useful is it? Not very. Modern ML models such as mobilenet, resnet or yolo require only a few hundred images for it to be accurate to solve those captchas.
You don't need few million samples, with 500-700 images per category you are more than ready to solve current captchas.
You don't need few million samples, with 500-700 images per category you are more than ready to solve current captchas.