Actually, their substantial diversification beyond Windows means that it's very, very unlikely Microsoft is going to fall (easily or quickly). Microsoft could lose the entire consumer Windows business and be just fine (despite the hit their stock would take), particularly as they keep growing in other sectors while Windows becomes less important by the day (and is clearly done growing as a business). Consumer Windows is about 1/3 of Microsoft's profit these days.
Apple by comparison gets about 3/4 of its profit from the iPhone. Google is even worse, 95% of their profit comes directly from search.
This is fundamentally incorrect. Only Online Services Division (Bing) loses money. Everything else makes money, and Windows definitely makes more than "a little bit."
Operating Income by Division FY13 Q3:
Windows: $3.46 B
Server and Tools: $1.98 B
Online Services: -$262 M (loss)
Business: $4.1 B
Entertainment and Devices: $342 M
It is interesting that people perceive so many businesses at Microsoft as "failing" even when they have healthy income and growth year after year.
Even though Xbox (Entertainment) is now making a small profit, they've got a long, long way to go before it recoups the vast investment/losses of the early days:
Apple by comparison gets about 3/4 of its profit from the iPhone. Google is even worse, 95% of their profit comes directly from search.