It appears that Google is trying to rewrite the rules of how browsers and the Web work, with the appearance of being on the side of privacy, but actually introducing an alternative method of surveillance that is going to be less favourable to almost everyone except Google. How many of the huge-audience sites are potentially going to lose out from that, not least because they rely on advertising themselves for the lion's share of their revenues?

This whole discussion started with a proposal from a platform that is supporting nearly half of the sites people are visiting. That puts WP in a unique and potentially very powerful position here as well, and evidently they're interested in trying to force the issue.

And finally, the SOPA experience has shown that it is not entirely implausible for large numbers of sites to collaborate in this way if they feel the threat is serious enough. So if FLoC is as bad as the critics are suggesting, it doesn't seem entirely out of the question. There seem to be quite a few powerful organisations that would have a variety of motivations for wanting to give Google a bloody nose over this one.