I would love for something like that to happen, and given all of the activity in the blockchain space, someone will probably come up with a clever solution. I just have a hard time imagining a motivation for Google to switch to using such a thing even if it were invented. They way it works now benefits them.
But even just looking at it from a purely technical perspective... If CAPTCHAs are intended to be "completely automated public Turing test to tell computers and humans apart", then its quite clear that they no longer accomplish this. I am a human and I can't pass. The model is wrong, build a better one.