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.
WEI randomly fails, website sees it, has never implemented any error checking (or fails on purpose without WEI), WEI becomes effectively mandatory.
Google is a gun manufacturer telling people on the other end of it "don't worry, every one in 20 bullets doesn't fire".