You're arguing worst-case, and people running anubis are saying it dramatically helps in their experience, because they're not getting too many of the sorts of bots that people like you and Tavis (correctly) argue are theoretically possible. So the argument that it's "bad CS" falls flat.
A better, perhaps the best, argument is that a worse bot mix might appear tomorrow, so anubis should be used as a runway for a better solution, and not as a solution in itself. If only everyone with a website had the desire (or AI tokens) to implement their own better solution. In some cases even desire or a lot of tokens isn't enough. I mean, you think kernel.org folks haven't put some serious thought into it? And yet, as HN argues about it, there's kernel.org running anubis at difficulty 5.
I don't know what you mean by "the best argument". My entire argument is that it's bad CS. I'm not trying to convince people not to run Anubis. If anything, I'm trying to convince Anubis to use better computer science. That would make more people run it! But even that's just a faint subsidiary goal; the real goal, as someone who works in this field and has designed proof-of-work antiabuse systems before, is just to add clarity about what can and can't work and why.
I too would love anubis to use better CS, if available. Perhaps you have some ideas for @xena, instead of just letting anubis continue on the trajectory of iterating its PoW approach?
A better, perhaps the best, argument is that a worse bot mix might appear tomorrow, so anubis should be used as a runway for a better solution, and not as a solution in itself. If only everyone with a website had the desire (or AI tokens) to implement their own better solution. In some cases even desire or a lot of tokens isn't enough. I mean, you think kernel.org folks haven't put some serious thought into it? And yet, as HN argues about it, there's kernel.org running anubis at difficulty 5.