This is true but so are the existing captchas. Existing captchas are just harvesting training data for Google's self driving vehicles at this point.
With a PoW captcha, it doesn't matter how smart you make your algorithm, it's still going to be slow. With existing systems I'd argue it's probably a lot slower for people than for machines, especially since it's people guessing what a machine thinks people would classify an image as.
This is an easy solution for rate limiting low trust/high risk connections and better software isn't going to magically make it any faster. This has always been what captchas aim to accomplish.
