And, the reality of your fancy idea is that normie users would turn away if they made a mistake on the CAPTCHA and were suddenly presented with a screen "charging" them one pence.
It's an entirely different sort of system, and it would require a cordoned off section of the Internet to implement it top-down, but it's technically viable.
The defining insight here is how many orders of magnitude difference there is between the "That price is negligible" threshold for a human being, and the "That price is negligible" threshold for an automated system. Sure there are adoption issues, but for all applications where there are several orders of magnitude difference, such a system makes some degree of sense.
According to a random page on internet [0], companies pay in $2-$6 range per 1000 ad impressions. If one pays $0.01 to bypass captcha and just 10 people see the resulting spam post, that's already $1 per 1000 views - much less than facebook charges. This becomes even more lucrative if the ads are expensive or there will be more than 10 people looking at the ad.
It looks you'll want much higher costs than that, which will make it "too much" for other users.
[0] https://spideraf.com/learning-hub/what-is-the-average-cost-p...
Another outcome that I could never understand: The original conversation was micropayments for traditional print media that was moving into the digital age. Why didn't they all band together to create an industry standard that defined (and possibly administered) a micropayment system? In the end, paywalls were the solution, and winner-mostly-takes-all when print moved to digital. Look at the decline in medium to small newspapers in the last 20 years in the US. It is devastating, but a few national, major newspapers are doing OK.
And I would argue we did get those in the form of subscriptions in Patreon, Onlyfans, Buy Me A Coffee, et al, or in the co-op world of Nebula. We didn't get them down to very low fee structures because we've designed our payment infrastructure with the intent of supporting a profitable company called Visa, Inc, to which we've offloaded a number of different functions of that a government mint / treasury / post office would normally perform. And because lots of revenue on these sites comes from whales, people with outsized income in a country with a great deal of wealth inequality.
What I am talking about is TINY micropayments just for human authentication purposes. Because what we've had so far in the realm of, for example, spam email, involves sending off messages at a CPM of less than a tenth of a penny. Imposing infrastructure which pegs human authentication tasks, normally performed less than ten times a day, at a CPM of ten dollars, can eliminate most applications of automated systems and eliminate the annoyance of captcha, while costing the human less than ten cents. There are no whales in the login space.