When it comes to the operating systems that power traditional PCs, Microsoft's (NASDAQ:MSFT) Windows remains the undisputed champion. Windows, in all its various forms, is installed on more than 90% of world's desktop and laptop computers.

Could that change? Apple (NASDAQ:AAPL) and Google (NASDAQ:GOOG)(NASDAQ:GOOGL) are beginning to take advantage of their mobile dominance, using the mammoth market shares of Android and iOS to bolster the relative strengths of their respective desktop operating systems.

Although this trend remains in its infancy, the growing power of Apple and Google's mobile ecosystems stands as a long-term threat to the continued viability of Microsoft's operating system.

Yosemite's new features
Earlier this month, at its Worldwide Developers Conference, Apple unveiled the next version of its Mac operating system: OS X Yosemite.

Yosemite includes a number of helpful features, many of which will benefit all owners of Apple's computers. But not everyone will be able to take advantage of everything Yosemite brings to the table -- in order to do so, Mac users will also need to own Apple's mobile devices.

Collectively, Apple refers to Yosemite's iOS integration as "Continuity."

Continuity is composed of a number of individual features that allow Apple's iPhones and iPads to work well with Apple's Macs. Examples include Handoff (allowing iPhone owners to do things like compose an email on their phone, then quickly finish it on their Mac) and call integration (letting Mac owners receive and respond to phone calls from their computer).

Many owners of Apple's Macs also own Apple's mobile devices, but many buyers of Apple's mobile devices don't own a Mac. According to a recent survey from Consumer Intelligence Research Partners, just one-quarter of the consumers who recently purchased an iPad owned a Mac, and only 28% of iPhone buyers had an Apple-made PC.

Given the relative market share of Microsoft's Windows, it's likely that many iPhone owners use a Windows-powered PC. As Continuity rolls out this fall, and Apple continues to support it, Windows PC owners that have an iPhone or iPad could eventually be enticed to make the switch.

Google has its own version of Continuity
Google unveiled some Continuity-like features of its own at its developers conference on Wednesday. Chromebooks, the laptops powered by Google's web-dependent operating system, will soon be able to interface with nearby Android handsets.

Like Apple's Macs, Google's Chromebooks will give their owners the ability to answer calls from their laptop. Chromebooks will also unlock instantly when they detect that their owner's Android handset is nearby.

Google won't be able to match Apple's Handoff right away, but intends to eventually offer something similar. Google announced that it will bring Android apps to Chrome OS at some unspecified future date -- and when it does, apps installed on the Chromebook could easily interface with the apps on the owner's Android-powered handset.

Microsoft's mobile efforts continue to struggle
Microsoft hasn't announced anything like Continuity for Windows Phone, but even if it does, it may have a difficult time driving adoption.

Microsoft has been supporting Windows Phone for years, but despite spending billions, has relatively little to show for it. According to research firm Canalys, Microsoft's smartphone operating system powered just 3% of the phones sold worldwide in the first quarter (Apple's iOS took 16%, and Google's Android accounted for 81%).

If it had pushed something like Continuity years ago, Microsoft may have been able to drive Windows Phone adoption. As it stands, smartphones appear to have overtaken traditional PCs in importance -- according to ExactTarget, 85% of smartphone owners say that mobile computing is a central part of their day-to-day life, and they tend to use their smartphone an average of 3.3 hours per day.

It won't happen overnight, but the increasing integration between Apple and Google's mobile devices and their desktop operating systems stands to put further pressure on Microsoft's Windows. Unless Microsoft can gain a foothold in the smartphone market, Apple's Macs and Google's Chromebooks could slowly chip away at Microsoft's Windows.

This article represents the opinion of the writer, who may disagree with the “official” recommendation position of a Motley Fool premium advisory service. We’re motley! Questioning an investing thesis -- even one of our own -- helps us all think critically about investing and make decisions that help us become smarter, happier, and richer.