Can I also suggest a quality-of-service monitor, say once/twice per minute from outside the site, or running internally but through a proxy? I think Discipulus and/or choroba, possibly other monks, developed something like this. Once the quality drops, record the ips from the logs and add them to the ban-list. It remains to be decided what kind of ban that will be, given all the points you made earlier.

Apropos captcha, perhaps PM can, once more, innovate by using perl-programming-related puzzles suggested by monks. So that AI bots don't leave empty handed but improve a little. After all, the use of Perl by AI would be much more influential for Perl, than by humans who seem to have, in their proverbial ignorance, abandoned ship :(. I can see these captchas soon to be replaced by whole PM SOPW questions that the AI must answer in order to gain temporary access to SuperSearch. Put those bots to real work.