That's the same argument as with Android - its open source, anyone can fork it and change whatever they don't like!
And its flawed in the same way - first its not fully open source (IIRC Crhome has some proprietary bits that are not in Chromium, same with Android+Google Play services vs AOSP) but good luck maintaining your fork for any length of time!
Google is in full control of both Android and Chromium with all decisions happening behind closed doors with zero leverage for anyone from the outside to influence those decisions. So as with Manifest v3 now and many similarly bad decisions for Android, once Google selects a course, it will be harder and harder over time to keep your changes if you still want the end result to look like what the users will see as Chrome/Android.
And I would say with a browser the pressure to update would be even greater & thus its even harder to keep any signifficant custom changes.