The problem is, Microsoft and the world don't particularly need an IE9 running on the latest and greatest OS, because there are
many excellent, modern choices: Chrome, Firefox, Opera, Safari, etc. Microsoft is kind of wasting its time making an IE9 that focuses only on the latest platform.
On XP, Microsoft could add tons of value by supporting a solid IE9 update. This is where browser improvements are truly needed. Let that be where their engineering talent goes.
You're right that forward-looking IT groups are already well aware of how to deploy a modern browser. It's the other companies I'm wondering about.
Consider what would happen if Microsoft announced these plans, and basically said that in 6 months they would start deploying IE9 by default on XP "for security" (which would basically be legitimate). Once both IT managers and contractors alike can see Microsoft's plans, the writing would be on the wall, like a mini-Y2K; there would be plenty of people you could call to have your broken IE6-only web apps updated, probably enough to keep the prices reasonably low. Obviously I'm speculating on that, but my point is that Microsoft is in the best position to set this ball rolling. A lot of the problem could solve itself. Putting IE9 on another OS isn't really going to tempt anyone using these ancient PCs.