If you the amount of features in Chrome and Firefox (just those in the standard, nothing extra), you would know "mature well to compete" is a long way away, if not impossible.
And I don't see any problem with forking. Tons of browser bugs were found, reported and fixed exactly because companies forked them. And remember that Blink is forked from Webkit.
I have seen IE's rise and fall. Netscape's rise, burn and rebirth as Firefox, saw Safari as a fork of KHTML and rise of Chrome.
Ladybird might be added to this list. It's not impossible. It'll be a winding and hard road to go, but it is not a path with no end.
You don't need to fork a codebase to fix its bugs. It's GitHub's workflow (fork -> PR -> merge). What I meant, as noted in this thread, is a hard and closed fork propelled with money and corporate greed, which eclipses the open and primary version and drown it in the process.
EEE'ing it, basically. This is why I prefer GPL (preferably V3+). If you want to improve it, it's open. If you want to monetize and EEE it, then nah. It's not allowed.
And I don't see any problem with forking. Tons of browser bugs were found, reported and fixed exactly because companies forked them. And remember that Blink is forked from Webkit.