That's a 12 foot dish covered with mirrors. It gets HOT
Microsoft co-marketing funds are a substantial portion of the profit margins for many large PC retailers. Tweaking the nose of the giant might be fun but it's risky. If Microsoft reduces the per-PC marketing contribution it makes for a particular reseller, that puts them at a huge financial disadvantage relative to their competitors.
It's not the costs of the OS that are the issue, it's the fact that Microsoft may take their ball and go home. Once that happens, you could be stuck with narrower margins, even if you're saving on the OS.