A few little problems with your assertion..
1) It's only in IE9. A browser that has yet to appear outside beta. IE's market share figures comprise of IE6-8 currently, and IE is not as quickly updated as other browsers such as Firefox/Opera/Chorme.
Not as big a threat as you think.
And will not be likely to see significant uptake until it is released as part of an OS. So what.. three-four years yet? A lot can happen in that time.
2) IE9 is going to be Vista/7 only. So again, a majority sector of the Windows platform left out. With full adoption today of all computers that are capable of using it.. 40% tops. So less than Firefox, but more than Apple's single digit browser share. Perfect adoption is however, unlikely. Because of all the people who use windows, about half or more use a non IE browser. So maximum number of IE9 users would be closer to 20% Add another 10%(being generous) for Safari,and you have a minority market share of browsers who will be compatible with the same codec. HTML5 compatibility on the other hand.. With these two?
3) IE's market share world wide is going down, has been dropping for months while others are all going up or staying the same. Chrome in particular is showing steady growth. If you check it by country with a site like StatCounter, the last month has seen pretty dramatic drops in IE use all over the world. And corresponding rises in Firefox and Chrome.
4) While IE as a whole may still have the largest share, depending on location, it is what it doesn't have that is still very important. Firefox started gaining significant share once it became too big to ignore. About 10%? So any site that decides to use HTML5 will not see it as a good idea to ignore 30%+ of the browser traffic. And as the sticking point is only the video aspect, not a huge deal. They can still use Flash or what ever for streaming video.
5) Silverlight.. Great success eh? How come it hasn't replaced Flash already? IE support is not nearly as much a guarantee of success as it used to be. Especially when some high profile Silverlight sites have gone back to Flash.
In reality.. this is a committee decision. And all the major players have representation there. Not a battle of market share. And the open internet side has a very significant presence.
MS and Apple agreeing should scare the hell out of everybody who wants an open web. So expect the iFanboys to forget who made it possible for them to use the internet without IE, and go for Mozilla next.