Not really. If Microsoft disappeared you could keep using its software as it isnow. And it will rot much faster than you can even possibly think about porting your solutions to a different platform. Seriously, would you use today an OS as it was originally developed - say - five years ago (as in no patches nor service packs)? What about developer tools?
Also, I don't know how you calculated that multiple, and I don't have a figure to give. What I know, though, is that supporting and maintaining any non trivial software over its entire lifetime often costs several times the original cost of development; I would be surprised if Windows were any different.