Competition in open source is not bad. It is actually needed. People can fork WebKit but they might not able to change it dramatically if it's not in favor of Google, Apple and Nokia.
I can't think about a case that WebKit "bosses" won't like a change from another party, but that might happen. We need competition for implementations. What we don't need is double standards for the web.
Microsoft proposed and standardized CSS grids which is awesome, but Google and Apple for some reason do not implement it in WebKit. Microsoft will not do that too. Both ends are enjoying the situation. Pulse.com works great in IE10 because of CSS grids. Microsoft can put their "work best in IE" logo on their website again. Webkit seems don't care much about CSS grids because they think flex-box is the solution.
This is the problem. We have companies making and implementing new standards for their use without caring for the rest of web.
> I can't think about a case that WebKit "bosses" won't like a change from another party, but that might happen.
Such things already happened, for example Google wanted to add changes to WebKit to support another VM in the browser (for Dart). Apple devs blocked the attempt for technical reasons, but some speculate political ones were relevant as well.