People have ran clones of the site with no captcha and effectively no rate limit, they go pretty much how you'd expect: some people play normally, some people bot massively. Part of the reason this captcha exists on OWOP is because bots not only mess up art and other creations, but also lag the server with heavy usage.

The sorts of people getting captchas are the sorts that would actively avoid "web attestation." It's more likely that they'd simply be prevented from using the site entirely.

Are you sure it would? How hard would it be for spammers to buy a Windows machine and run Chrome?