And why should they?
Microsoft is already walking this path with their Office 365 subscription model. But as long as the common Office products bring in money then I'd say Microsoft would be a fool to whack that simply in an attempt to gross in more money. I doubt even they are that stupid (but I have been proven wrong before on those opinions, so who knows...).
Personally I think Microsoft couldn't afford a move like this, not right now while they admitted that their latest "flagship" has indeed entered some rough weather.