That's a very good point. IE9 has 3% usage share right now, but because it can't run on XP, IE9 won't replace all the IE8 installs out there. In the same vein, IE10 won't even run on Vista. That's why I doubt we'll see another single version of IE being the most dominant. On top of that, Chrome usage is growing rapidly. At the current rate, Chrome will be the most popular browser in 12 to 18 months.
Chrome and Firefox are the browsers that supports html5 video with the biggest market share though.