What everybody seems to be forgetting is that there is a ton of web-content fetching being done right now which is not done by browsers.

For example, all the webcrawlers doing indexing for search engines.

Consider the small possibility that any major website that does this either becomes inaccessible for any webcrawler which does not implement this (say, those indexing sites for search engines other than Google's) or has exceptions for webcrawlers which are one big backdoor for browsers to also come in (in fact a number of paywall-bypassing solutions relly on sending the right HTTP headers to use exactly existing exceptions for webcrawlers).

Even webcrawlers implementing this are relying on "integrity validation" servers from a 3rd party (I bet that's going to be Google) so think about how Google can interfere here with 3rd party webcrawlers by merelly throttling down integrity validation responses for those.

Oh, and open source webcrawler implementations can forget all about being validated.

(By the way, this would also impact any AI-data gathering webcrawlers that don't use APIs to get the data but rather go in via the web interface)

This is quite possibly a far bigger play by Google than just for browser and Ad dominance.