Does Office need the R&D cost of Windows weighing it down ?
It's all about the upgrade cycle and differentiation. I don't think r&d costs for windows weighs much unless you think marketing is r&d.
If office and visio and project were available on Linux / osx you'd probably see a drop off in windows revenue for no office revenue gain. If you need those apps, you pay the "line rental" cost of windows. That cost is there to make sure you don't also commit to another platform.