- there is a viable alternative to Microsoft Office on the desktop. This doesn't mean one that gets most of the way there; LibreOffice and company, Office on mobile, etc. don't quite cut it. Macro jockeys and VBA scripting are huge game changers for corporate clients, especially in the finance and accounting worlds.
- common CAD software has been entirely ported to a new platform and has complete feature parity. Considering how conservative engineering/manufacturing firms can be, this will likely never happen. No large engineering corporation will ever risk, or run the risk of ever having to run the risk, of even the slightest of hiccups in a large software migration. The only big swinger even making an attempt in this department is Siemens NX, and even we're still quite far out from feature parity for even that single package.
- Active Directory has a viable counterpart for a competing operating system. Some people are trying, none are truly succeeding.
Those three points alone are enough to prop up Windows for the next couple decades. Don't get me wrong, I'm not a fan of Windows, Microsoft, their business practices, or many of their ideas on how things should be done. However, it drives me up the fucking wall when people make dumbass claims that Windows will die within any reasonable timeframe (20-30 years).
EDIT: As a quick clarification, I bring up CAD not because it attracts a large userbase (it doesn't), but because switching away from Windows means that the manufacturing base of the entire world will undergo a massive change that could prove fatal in many ways. This means that any company with a significant CAD userbase will be using Windows, even if the HR, IT, etc. doesn't, in an attempt to consolidate overhead. This also means that many companies that work with engineering/manufacturing companies will stick with it for compatibility's sake. Many people are commenting that CAD/manufacturing doesn't account for many users, but that's beside the major implication. Engineering and manufacturing customers alone could ensure at least a workable level of Windows support, all while indoctrinating most of their employees in to Microsoft's ecosystem.