You may have noticed the quotes I put around "from scratch" in an effort to forestall responses just such as yours. Yes, I realize that they built on a substantial amount of existing work, but every project does. The point is that they did nothing that nobody else could do, there's nothing stopping MS from using WebKit in IE9. The fact that a relatively small number of total developer-hours compared to the efforts having been expended for development of IE7 and IE8 can translate into a top-tier web browser is significant.
The Internet is such a terrible medium for communications subtleties. I agree though... but imagine how that would look from a PR perspective. Microsoft is, essentially, the anti-thesis of open source.
