Everything Microsoft does is about the domination of Windows and Office. These are their big moneymakers and they don't want to give them up. Are you sure that a royalty of $50 for practically every single PC manufactured in copyright-observing nations is worth less than their enterprise software? What about everyone that goes out and spends $100+ on a copy of Office? Those are some big markets and I'm not sure their value is so diminutive.

Additionally, such a platform allows tight integration of things like .NET, attracting developers to Windows' platform and allowing MS to sell dev tools and servers. Would .NET be worth less if Microsoft's desktop share were diminished? Almost definitely, imo.

Please don't underestimate the value of desktop dominance.