Google's crawlers don't exactly perform 2FA before they crawl a website. If impersonating Google's crawlers doesn't suit your fancy, there are all manner of ways to anonymize a crawler. So I think blocking is one of the least interesting challenges. The other side of the coin is robots.txt is not inherently adversarial, and a crawler could waste quite some time and energy crawling truly meaningless content. That, in my mind, is the interesting challenge: Obsoleting robots.txt with an intelligent (and gentle) crawler.