Their proposal is that, when you visit a website using WEI, it doesn't let you see it right away. Instead, it first asks a third party if you're "legit", as opposed to maybe a bot or something.
The problem is, it would be really tricky to tell if you're "legit", because people get very, very tricky and clever with their bots (not to mention things like content farms, which aren't even bots, they're real humans, just doing the same job as a bot would). So, in order to try to do their jobs at all, these kind of third parties would have to try to find out a whole bunch of stuff about you.
Now, websites already try to do that, but for now the arms race is actually on our side; the end user has more or less full control over what code a website can run on their browser (which is how extensions like u-block and privacy badger work).
But if the end user could just block data collection, the third-party is back to square one. How can they possibly verify ("attest") that you aren't sus, if you're preventing all attempts at collecting data about yourself, or your device / operating system / browser / etc?
The answer is, they can't. So, to do a proper attestation, they have to have a whole bunch of information about you. And if they can't, they logically have no way of knowing if you're a bot. And if that's the case, when the third-party reports that back to the website you're trying to visit, they'll assume you're a bot, and block you. Obviously.
That's pretty much my understanding of the situation. In order to actually implement this proposal, it would require unprecedented invasive measures for data collection; and for people who try to block it, they might just end up being classified as "bots" and basically frozen out of major parts of the internet. Especially because, when you consider how people can essentially just use whatever hardware and software they want, it would be in these big companies' interests to restrict consumer choice to only the hardware and software they deem acceptable. Basically, it's a conflict of interest, especially because the one trying to push this on everyone is Google themselves.
Now, Google obviously denies all that. They assure us it won't be used for invasive data collection, that people will be able to opt out without losing access to websites, that there won't be any discrimination against anyone's personal choice of browser/OS/device/etc.
But it's bullshit. They're lying. It's that shrimple.