I was going to write, that MS did not went out of their way to make developing alternative browsers, but then I have changed my mind.
How big Win8 can be in its first Q out in the world? And than how bigg Win8 Modern will be? (or whatever MS call Metro today).
There is also one more thing. Win8 will be spearheaded by early adopters. They are usually more tech aware than most. So they will check which desktop environment offer better browsing experience. And since JS JIT will not be available for “Modern” …
(And even if MS will make JS fast enough to be competitive it will not be for long …)