Micosoft made its fortune off of the Desktop market.
Windows, and Office. + The slue of apps that support the two. Programming, Servers, IE...
Now not everyone wants or needs a desktop.
They didn't get much effort in getting Mobile. Zune, Windows Phone, the PC makers are kinda floundering on Windows Mobile tablets.
Their XBox gaming is a fickle market. They are in way too tight race with Sony, then you have the mobile market taking up a lot of the indie game market. Screwups like they did with the XBox One launch can cause major issues. Forcing people to choose an other gaming system before the release.
Having the vendor lock in, just isn't working... Too many Rich HTML web applications out there, meaning people are not even caring if they are on Microsoft Server of LAMP.
In order for Microsoft to last for the future they will need to be more Open. So those .NET apps work in Linux and Windows, So people who care about the App that it runs not the OS (Like most people, just not Slashdot) means they will not need to switch to an other platform. If they keep on windows only. The fact people will feel stuck may mean they will chose a more open app,
I like my house, I like to stay inside my house... However if I feel like I am stuck in my house I will want to leave it.
Making microsoft open and allowing a way out, means people are not coming up with reasons to leave.