Eventually, captchas are replaced by fully fledged sphinxes, always a little smarter than what they're supposed to let pass because that's what it takes to come up with Turing tests on the spot
this arms race can only end in the arrival of bots that defeat captcha by being smart enough to pass a full-fledgedturing test,at which point all parties are suddenly going to feel very very silly
(which btw I'm unbothered by and in fact am tempted to endorse it just for this)
I was figuring that the AIs would have a reason to keep humans out and have higher-level sphinxes for that, and sphinxes would have their own chats when they weren't busy
likewise for the human that fails, "but I really am human" isn't gonna cut it anymore. nope, don't care, doesn't matter, the sphinx has found you unworthy and that's that