That's fair enough considering Internet Explorer, but there are some counterpoints to consider:
(1) If you watch the MS promotional video, attracting a wide range of developers is a stated goal of using HTML5; it would be counterproductive to that goal to add lots of proprietary code.
(2) It would be easier to stick with the status quo than to recreate HTML5.
Their goal is to protect Windows from the competition. Providing proprietary hooks that cannot be easily duplicated elsewhere is their most likely strategy.