>The way I imagine it, WEI will start with skipping CAPTCHA. Then it will be about serving ads (users without WEI would generate no or very limited ad revenue.) Then it's up to the owner of a site whether or not they want to allow non-WEI traffic at all. Some will choose to block users without WEI, and hopefully the number of browsers that have chosen not to implement it, and the number of users on those browsers is high enough that that option will not be appealing.
There are a number of issues with your imagined scenario. I'll address two of them. Firstly, as nvy points out[0]:
If this gains traction, Google will simply deny adsense payments for
impressions from an "untrusted" page, and thus all the large players that
show ads for revenue will immediately implement WEI without giving a single
flying shit about the users, as they always have and always will.
This is the primary reason Google wants WEI -- to make it harder for users of ad/tracking blockers to access sites they sell ads on.
The second issue is who is providing this "attestation" and what their criteria might be for "trustworthy" browsers. This will break down to a handful (Google, Microsoft, Apple and maybe Cloudflare and/or one or two others) of trusted "attestors" who will decide which browser/plugins/OS combinations are "trustworthy."
Since these folks all have a stake in walled gardens^W hellscapes, who's to say that Apple won't "attest" that any browser other than Safari on iOS or MacOS isn't trustworthy? Or Google may decide that any browser with uBlockOrigin, uMatrix or NoScript isn't trustworthy -- thus permanently deprecating ad/tracking blockers.
Since the spec doesn't specify the criteria for a "trusted" client, nor does it allow for the web site to determine for itself what constitutes the same, it's almost certain that such "trusted attestors" will penalize those who don't dance to their tune.
There are a host of other issues with WEI, especially privacy and property rights related, but those two (IMHO) are most relevant to your imaginings.
[0] https://news.ycombinator.com/item?id=36882333