Regardless of what happens to Chrome per se it's who is involved with pushing for major controversial changes in Chromium that matters.
Manifest v3 and Web Integrity API are prominent examples of Google's team shaping how all Chromium based browsers will be, regardless of pushback (though they relented with the latter for now).
Manifest v3 is not even breaking any standards. This is like saying Google should not make any changes to their browser as any forks will not be able to maintain any divergence. All forks are free to keep manifest v2. Off course maintaining a browser is expensive, but that doesn't mean Google has to foot the bill for everyone and everything.
In theory, yes. In reality, the more diverged forks become from mainstream the more expensive they become to maintain, until eventually it becomes entirely unsustainable. With the sheer number of Chrome patches Google churns out, the level of divergence where maintainence becomes overwhelming is actually pretty low. It’s like trying to handle Niagara Falls with a Solo cup.
By that logic google can not make any changes to their browser unless all forks agree to it, as the forks will definitely not be able to continue maintenance for long. By this logic it makes more and more sense to not open source anything and keep your product as tightly closed as possible (what Apple/MSFT have been doing).
It’s explicitly changes that are a net negative for users and privacy (e.g. manifest v3) or try to circumvent the web standards process (e.g. WebUSB) that are problematic and would be of interest to forks. Most changes are fine.
Of course, we should ask why the web is in a place where building a browser is so massively complex and expensive that Google and Apple are the only entities in the world who can afford to do it.
No motivated group has the experts to maintain a modern rendering engine (tens of millions of lines of regularly evolving code) and Google can reject any upstream changes that group wants to make because Google engineers are the gatekeepers for 97.498% of Chromium code. So, if your agenda has anything to do with web content, you take what Google hands you. Of course, if you just want to diddle in the browser UI, sure fork it, that's the Brave and Opera approach, but hardly meaningful in scale.
The big players have made the web so complicated that it's impossible to maintain one's own standards compliant browser without millions of investment, and you don't get a business model because everyone else is giving their out for "free".
This problem really needs to be fixed, though I have no idea how...
Web technologies enable server-client apps that can rival desktop apps, and Browsers are almost as complex as Operating Systems. What you describe isn't a problem, it's made like this because they're very useful and complex technology stacks
Why does a browser need to be 10,000,0000 lines though? It's that the minimum number of lines to capture the complexity of a modern browser? I don't it.
And even if there are 10mil lines they shouldn't be a monolith
That's not relevant. Chrome has the most market share, so its decisions become de facto standards. What happens in some fork by a 2 men crew doesn't matter.
Manifest v3 and Web Integrity API are prominent examples of Google's team shaping how all Chromium based browsers will be, regardless of pushback (though they relented with the latter for now).