(A secondary thing is that AI bots have basically zero benefit for most websites, so unless you are some low-cost crappy content farm, it'll be in your interest to raise the prices to the max so the bots are simply locked out. Which will bring us back to point 1, bots ignoring the headers)
Also robots.txt is a suggestion but hashcash is enforced server side. I agree it's a tragedy people have started to completely ignore it but you can't ignore server side behavior.
AI bots are not search engines, and they have no benefits for the website owners. This can be very clearly seen because they ignore robots.txt, pretend to be regular browsers, and use multiple proxies to avoid IP bans.
...and the requisite checklist: https://trog.qgl.org/20081217/the-why-your-anti-spam-idea-wo...
This would make the entire internet a maze of AI-slop content primarily made for other bots to consume. Humans may have to resort to emailing handwritten PDFs to avoid the thoroughly enshittified web.
At the moment, ad networks don't pay for bot impressions when detected - so content farms tend to optimize for what passes for humans. All bets are off if human and bot visitors offer the same economic value via miners, or worse if it turns out that bots are more profitable due to human impatience.
Imagine an internet optimized for bot visitors, and indifferent to humans. It would be a kind of refined brainrot aimed at a brainless audience.