The thing that keeps Microsoft afloat is its Windows monopoly
(and the Office suite and servers, but both are strongly tied to Windows as they have little presence elsewhere). Up until 10 years ago, the only threat to Windows was Mac OS which had been stuck at 5% market share for decades, and Linux for the desktop which lingered around 1%.
The last 10 years have seen two new entrants to the operating system market - iOS and Android. iOS still has a relatively small, but lucrative userbase. Android already matches if not exceeds Windows' installed userbase. A lot of people dismiss these as a "toy" OS for toy devices. But that's being ignorant of the march of technological progress. 30 years ago my primary computer was a desktop. 20 years ago it was a laptop that was nearly 2 inches thick. 10 years ago it was a notebook just under an inch thick. Today it's a half-inch thick ultrabook, but about half my screen time is on my phone and tablet.
Mobile isn't going to go away. Eventually it's going to eat the laptop and eventually desktop markets. Intel charges a fortune for their CPUs (about $100-$1000 vs about $5-$20 for ARM). As technology advances and ARM processors become more and more capable of performing everyday computing tasks, there will be less and less reason to spend an extra $100-$500 for an x86/x64-based "computer". And if x86/x64 dies, Windows dies with it. Microsoft knows it, and its shareholders know it.
That's why Microsoft worked so hard on Windows RT (basically Windows for ARM). That was their warning shot across Intel's bow that they had better do something to stave off the advance of ARM devices, or Windows was going to jump ship and abandon x86/x64 for ARM. It worked. Intel came out with some new extremely low-power CPUs which were almost competitive with ARM in power consumption but ran x86/x64 software, thus slowing ARM's encroachment into the laptop market (e.g. early Chromebooks were ARM, but they're now Intel). At least for now.
But the Intel can't keep it up forever. Their tax is very high per cm^2 of silicon compared to ARM. Eventually they're going to have to cut their prices, or ARM is going to win out. And if ARM wins, which OS do you think is going to dominate? Windows RT? Yeah neither do I. Which is why Microsoft's shareholders are so anxious that Microsoft do something, anything
, to gain a foothold in the mobile (ARM) market.