If it's about using only Google's services, then yes I agree, but the point is if lots of random sites all decide to use Google for captchas.

This has already happened with tor and Cloudflare, but at least that changed for the better recently (see https://www.zdnet.com/article/cloudflare-ends-captcha-challe...). In that case it was just one CDN using captchas to discriminate against a group of users, so that one change by the CDN could fix the issue. If too many random sites are independently blocking or slowing down anyone not logged into Google, then that'll turn the web into Google's web.