As devs we don't necessarily want a single browser to be dominant even though we might jokingly pray for such a thing during testing phases. We just want them to be standards compliant, right?
Don't get me wrong: there is a certain freeing joy when you work on a project where you know the target browser is the only one you need to think about (like say if you build kiosk apps or apps on a strict intranet).
But at the same time, if one player is too dominant or we only have a single voice about "this is what a browser should be" we get the crap we have lately with Apple's voice being too strong on what is or is not accepted into the HTML standard; or when IE had the 90%+ share it used to have.
Well, let's flip the question around -- why are there three major open-source rendering engines in the world right now already? Why did Google fork Blink from WebKit?
The answer seems to be, major browser vendors have a hard time cooperating at the engine level. Apple and Google had different opinions over WebKit, which eventually led to it being forked. Microsoft has no real reason to think that it could partner with Apple or Google any more successfully than they partnered with each other in developing a rendering engine.
