That is not happening in what I described. A webpage could decide to offer the cheapest working captcha as a default with a "Are you using assistive technologies?" / "Do you need help?" link to a more accessible service.

I don't think that is will be common in the wild, but it is an argument against regulating captcha providers rather than websites.

Oh, I see what you mean. There may be a market for "accessible CAPTCHA", that's audio-only.