I don't know much about the online ad market. I assume advertisers will pay more for attested impressions than for unattested ones. But unattested impressions will still be worth something.
It's very simple. Google has concerns of click/impression fraud. Unattested traffic would be more likely to be fraudulent. Not paying for unattested impressions/clicks is therefore an easy way to cut costs and combat fraud.
Chrome will happily block a Google ad if it uses too much resources, I experience this a lot with a few sites that do ad replacements in the background.
Now if Google cares about real impressions it's still terrible no good very bad evil.
It's good for google to care, it's not good for them to do this.
Because this is an incredible way of exerting their total control over the web across all browsers. If they don't like a feature, they get to downgrade the user's attestation or fail it. If it costs them some unattested traffic in order to create a permanently unassailable market position, it's worth the money.
It'll block all other search engines by preventing web scraping except those blessed by Google. For this reason alone many websites will adopt it. This will impact competition, research and freedom.
After this, all user choice is gone, and it'll only be governments who can break the racket.
If the CCP don't already do this, I expect they'll quickly implement something similar.
I don't think Google has actually done anything. The bar for experimenting with new code in Chromium is pretty low. This Chicken Little reaction to a non-starter is just a result of developing in the open.
But you can "care" about something in good and bad ways, and the criticism is not "Google bad".