>Systems like Altcha put an end to this argument. They don't care if the browser looks suspicious, only that the browser can perform a proof-of-work to get past a captcha designed to slow down the request rate.

That doesn't really work out in reality because bots are happy to wait 5 seconds or even 5 minutes for a PoW challenge to complete. Humans on the other hand will not, especially if they're on a mobile device with limited compute and energy.