Unpacking Google’s Web Environment Integrity specification

I'm curious to hear from someone familiar with web development: How much do websites invest in accessibility and related features that cater to a small audience? Can we draw any conclusions from this to how websites will deal with accessibility to non attested users?

Depends on the company size, really.

Large companies will invest significant resources with us to achieve AAA compliance with WCAG 2.1

Smaller companies will spend SOME additional budget to achieve AA.

Tiny companies will spend nothing until they get a demand letter.