> Building on top of Chromium means that it takes mere millions. And that changes the situation.
This simply does not follow. If you're building on the back of Blink you're still chasing whatever Google unilaterally decides to include in Blink. You need to do extra work to merge stuff you want and keep stuff you don't want properly disabled. Google has no impetus to make it easy or even possible for third parties to disable features in Blink. The cost to maintain a defanged Blink can very easily go from "mere" millions to billions if Google makes it difficult to merge upstream changes in defanged forks.
If web developers readily adopt whatever Google throws out, and lets be honest it's adtech companies adopting "features" to better fingerprint users without cookies, then a Blink-based alternative to Chrome will get zero uptake. If the top sites on the Web require Google's version of Blink/Chrome with all of Google's handy dandy anti-privacy features then it does not matter in the slightest that a non-Google Blink browser can exist.
You're pretending that Blink being Open Source is somehow going to affect the decisions of web developers (adtech companies). They are going to chase Google's version of Blink/Chrome because that's how they make the most money. Right this second Apple and Mozilla are just barely keeping Chrome from fully dominating the web.
Google is never going to make Chrome overtly user hostile. They're just going to continue to making Chrome an advertiser's dream browser because they are an advertiser. While WebEyeTracking might have some non-advertising use 99% of the user cases will be to make sure people looked at an advertisement long enough. If Google controls the specifications that define the web and sites adopt those technologies, there's no room for alternatives that aren't Google's Blink. Not only can defanged Blink not be practical but neither are non-Blink browser engines.