But here we're talking about a system where legitimate users (human browsers) and scrapers get the same value for every application of the work function. The cost:value ratio is unchanged; it's just that everything is more expensive for everybody. You're getting the worst of both worlds: user-visible costs and a system that favors large centralized well-capitalized clients.
Based on my own experience fighting these AI scrappers, I feel that the way they are actually implemented makes it that in practice there is asymmetry in the work scrappers have to do vs humans.
The pattern these scrappers follow is that they are highly distributed. I’ll see a given {ip, UA} pair make a request to /foo immediately followed by _hundreds_ of requests from completely different {ip, UA} pairs to all the links from that page (ie: /foo/a, /foo/b, /foo/c, etc..).
This is a big part of what makes these AI crawlers such a challenge for us admins. There isn’t a whole lot we can do to apply regular rate limiting techniques: the IPs are always changing and are no longer limited to corporate ASN (I’m now seeing IPs belonging to consumer ISPs and even cell phone companies), and the User Agents all look genuine. But when looking through the logs you can see the pattern that all these unrelated requests are actually working together to perform a BFS traversal of your site.
Given this pattern, I believe that’s what makes the Anubis approach actually work well in practice. For a given user, they will encounter the challenge once when accessing the site the first time, then they’ll be able to navigate through it without incurring any cost. While the AI scrappers would need to solve the challenge for every single one of their “nodes” (or whatever it is they would call their {ip, UA} pairs). From a site reliability perspective, I don’t even care if the crawlers manage to solve the challenge or not. That it manages to slow them down enough to rate limit them as a network is enough.
To be clear: I don’t disagree with you that the cost incurred by regular human users is still high. But I don’t think it’s fair to say that this is not a situation in which the cost to the adversary is not asymmetrical. It wouldn’t be if the AI crawlers hadn’t converged towards an implementation that behaves as a DDOS botnet.