Any browser choosing not to implement this would not be trusted and any website choosing to use this API could therefore reject users from those browsers.

This is not a new problem. Banking and mercantile websites are notoriously browser-intolerant, and have been pressured by customers to adopt their websites to accept non-standard browsers. Considering that even Google ads will sometimes deliver malware or phish users into a malware download, adblocking remains a web-hygiene feature.

I could see big companies pushing WEI only to find them losing sales from enough people who can no longer use their websites to complete transactions. Amazon is really going to want to make sure non-WEI compliant users can still buy stuff.

And then it will be probably months before a team of engineers builds a gateway adapter that makes any computer system WEI compliant by making their preferred browser transparent, or look exactly like an optimal Chrome setup. Pissed-off Linux programmers are a force of nature.