Sure, the individual bots that adapt to solve the CAPTCHAs are stronger than before in the sense that there are pages they can crawl that their earlier versions

Sure, the individual bots that adapt to solve the CAPTCHAs are stronger than before in the sense that there are pages they can crawl that their earlier versions could not. But the subset of pages that they can crawl hasn't increased in an absolute sense: For any page that they can crawl now, the old (CAPTCHA-free) version of that page could already be crawled by the old (non-CAPTCHA-solving) version of the bot. So in that sense, nothing is lost by adding CAPTCHAs to pages.