Ie. on a given device, for 10% of websites, WEI pretends to be unsupported.
That means websites can't deny service where WEI is unsupported. Yet it still allows statistical analysis across bulk user accounts.
If WEI was implemented like this, I would support it as being good for the web ecosystem.
Will it though? Googles main reason for WEI I assume is to combat ad-fraud. Ie. to prevent someone making a bot farm to click ads to earn money from advertising or exhaust competitors ad budgets or manipulate search engine user ranking signals.
With WEI, all ad clicks without WEI could just be ignored (ie. not billed to advertisers, ignored when calculating statistics and signals). If 10% of clients have WEI 'cloaking', you just inflate the final advertising bill by 10% to account for those users - the end result is the same as billing for all real users and no bots.
WEI still achieves all of Googles goals even with cloaking.
Companies give google $X, and hopefully sell Y extra products. X/Y is the cost per sale. Google competes with other advert forms (eg. TV/radio/newspaper ads) on that X/Y number.
If there is ad fraud, that Y number gets decreased (budget is used up on fraud that doesn't translate to sales), and their revenue decreases as advertisers spend their ad budget on other mediums.