one “side effect” of this would be enabling Google to effectively block ad-blocking. And, of course, plenty of people will insist that that’s not a side-effect, that’s the end goal.

How would this block ad-blocking?

Websites could choose to deny access to clients that attest to having adblocking addons installed.

Point me to anything which would give websites access to that information via WEI. There is nothing. I have seen nothing except FUD. Aside from that, this only attests for the device. Ad-blockers can be external. This does nothing for external ad-blockers.

Explicit non-goals for WEI:

"Enforce or interfere with browser functionality, including plugins and extensions."

https://github.com/RupertBenWiser/Web-Environment-Integrity/...