Conveniently, you neglected to mention that IE9 and IE10 are FAR FROM having wide-spread adoption in the browser market.  According to w3schools.com IE has a total of 18.3% browser marketshare for 4/2012, and IE9/10 account for a COMBINED 6.5% TOTAL browser market share for 4/2012.  That means thats 64.5% of all IE usage is still IE6-8.

There in lies the problem.  It's not that Microsoft isn't baking in less suck than they traditionally have in older IE versions; it's that they have a distribution and fragmentation problem.  IE9 and 10 might be the best thing since sliced bread; but no one is using it.  

Furthermore, Microsoft has left a nasty taste in my mouth regarding supporting Internet Explorer.  Development tools up-to-and-including IE8 are garbage.  They're a lot better than older IE versions, but they're still WORLDS behind Chrome and Firefox.  I can't even debug any version of IE without a VM of Windows installed, which again is a giant pain in the behind.

Microsoft has several problems to solve in the browser game.  Primarily, making their browsers not be complete trash.  Secondly, FORCE USERS TO UPGRADE!  I don't care if your browser is the most performant browser in the world, if no one uses it - I'm not wasting me time. Thirdly, I NEED TO DEBUG CODE!  I can debug all the major browsers on my Mac with minimal effort; no I don't get complete coverage, but it's an excellent base.  I can't even LOOK at a site in IE without booting a VM, which Microsoft doesn't even *really* make available.