The problem has never been that some bots could eventually seem like they were human. The problem is and will continue to be that many humans (millions upon millions) look like bots.
Have you never once looked at the captcha and couldn't decide whether the 3 pixels of the motorcycle sticking out into the grid square meant that you should select that grid square too? Not once? As the tests become ever more sophisticated, more and more of you all will be locked out.
Or you'll get the "click all squares with a stop light" prompt and it's a closeup of a signal light so you just click everything... But if you get it correct _and_ too quick l, you're a bot!
Have you never once looked at the captcha and couldn't decide whether the 3 pixels of the motorcycle sticking out into the grid square meant that you should select that grid square too? Not once? As the tests become ever more sophisticated, more and more of you all will be locked out.