After a disastrous Windows 8 launch in late 2012, Microsoft reorganized its entire corporate structure, sent its CEO packing, adopted a rapid release cycle, and quickly shipped a mulligan in the form of Windows 8.1 in 2013. Problem solved, right? Nope. Windows has never faced adversity like what it will face in 2014. And this coming year will prove pivotal for Microsoft's flagship OS.

Related: Big Changes Are Coming to Windows

Can you guess what the biggest problem facing Windows is? Here's a clue: It's probably not what you think it is.

It's not the lackluster response to Windows 8. Yes, pundits, analysts, and apparently even the general public were routinely antagonistic about Windows 8, a system that was force-fed to an unwitting audience in 2012 so that Microsoft could quickly make up lost ground in the new market for mobile personal computing devices. But while most of these critics begrudgingly admit that 2013's release of Windows 8.1 fixed most of the issues they had with the original Windows 8 release, it's fair to say that even this improved update doesn't go far enough.

It's not Linux, which never emerged as a force on the PC desktop.

It's not Mac OS X, which despite strong sales growth over the past decade never hit double-digit market share worldwide, though Mac PCs sell strongly in very rich nations like the United States.

It's not tablets, though I'd pinpoint this latest move to simpler personal computing to be a major contributing factor: People realize that they don't just not need the complexity of Windows, they don't even need most of the power of Windows.

Windows is in trouble because people simply don't care about it anymore. It's not outright hostility; there's far less of that than the anti-Microsoft crowd would like to believe. It's ambivalence. It's ambivalence driven by the nature of "good enough" mobile and web apps. It's ambivalence driven by the allure of anytime/anywhere computing on tiny devices that are more cool to use and even cooler to be seen using.

And make no mistake, this is a serious issue. With businesses keeping Windows on life support and users spacing out their PC purchases for so long that there might never in fact be another PC purchase, Windows is in trouble. This ambivalence is worse for the platform than outright defeat. In its current state, Windows can limp along for years to come. And that's just long enough for the platform to wither and effectively disappear.

In the past, Microsoft would show up with a new platform—Win16, Win32, .NET, whatever—and developers would rally around it immediately because Windows represented the volume market for personal computing. The resulting applications would be purchased, downloaded, and used by real users, and the programming standards that developed over time—toolbar and button types, property sheets, and other ways of doing things—permeated across popular applications, creating a standard look and feel. Windows and its applications were comfortable, familiar, and popular. And then they weren't.

It didn't start with tablets, sorry. And it didn't start with Mac OS X. It started with Longhorn, the project that outgoing Microsoft CEO has (correctly) pinpointed as the biggest mistake of his tenure. Longhorn was the point at which Microsoft's ambitions exceeded its abilities. And it derailed Windows, and the company, for the better part of a decade.

Longhorn addressed the wrong problem for the era, and it did so with the backing of a set of all-new, .NET-based APIs that morphed throughout the years-long development of the platform. By the time Microsoft spat out Windows Vista, it was as top-heavy and unwieldy as the organization that created it. Worse, it fulfilled precious little of the original promise of the platform.

While this was happening, web apps, phones, and then tablets were becoming first viable and then truly powerful. While this was happening, developers stayed away from Microsoft's new APIs in droves and created absolutely zero major new applications with that technology. While this was happening, desktop applications such as Office, Photoshop, and iTunes lumbered along, more out of inertia than anything else.

Related: The Death of the Windows Desktop

What we're left with is a bizarre situation in which the "popular" part of Windows—the desktop, which is used by over one billion people every single day—has stagnated, with no major new application development in years. I reported previously that of the top 10 most frequently installed Windows desktop applications, two—iTunes and Chrome—are essentially rival platforms of their own that aim to steal away Windows users, while the rest are silly little utilities that fix problems with Windows 8. And the Modern (previously "Metro") mobile environment that Microsoft bolted on to Windows, although generally well-liked by developers despite its immaturity, is seldom used by actual users. That's because most Windows users are still using traditional, non-touch PCs. Which they might never upgrade.

Rock, meet hard place.

I coincidentally spoke to a friend who works for a major technology company that has a big presence in the Windows world. (I can't name the firm, sorry, but it's as well-known as, say, Adobe and has been around for many years.) After rising to fame and fortune on the back of C-based Windows applications a decade or so ago, this firm has seen its user base splinter, with many on Macs and many more on iPhones, iPads, and Android devices.

The company's engineers have little to no interest in Windows per se, and are gravitating to MacBook Pros. While they seem to actually like the "Metro" APIs from Windows 8.x, according to my friend, there is absolutely zero call for creating such apps. And their flagship Windows products are hard to maintain and update because of the lack of interest and excitement around Win32. They're looking at moving them to more modern, perhaps even open-source foundations.

Sound familiar? This song is being sung in companies around the world, where users are moving to Android and iOS mobile apps and to web apps. Apps tailored to these experiences are now at the forefront, and Windows, when it's considered at all, is on the back burner.

Again, it's not that there's any real "hatred" of or disdain for Windows. There's just a very real acknowledgment that the world is moving on.

How Microsoft addresses this change is unclear. We know that the firm in embracing a "devices and services" strategy is doing so agnostically, and we've already seen many high-profile Microsoft apps and services show up on competing devices this past year. I'd be surprised if 2014 passed without major, full-featured versions of Office on both iOS and Android.

But Windows, too big to fail, doesn't need to fall to a single competitor, and it hasn't. It just needs to be ignored. And that is, I think, the biggest worry for Microsoft in 2014. Not that Windows fails. That it simply fails to matter.