If that's your metaphore, then the browser engine is the kernel. Have you ever seen a Linux distro forking the kernel and taking it into a different direction? It's always just (relatively) minor changes, if any, so that keeping up with the upstream kernel is possible. Mostly a matter of built-in drivers, and a security feature here and there.

And what happens when your fork diverges too much? That will be maintenance nightmare, good luck with that.

That's over easy in the picture above. The fried egg in the picture hasn't been flipped - no way the yolk would've remained flawless like that otherwise.