It’s not a secret that Microsoft is in some significant trouble. Windows 8 has been deemed a failure. So has Microsoft’s foray into hardware, the Surface. Still, it wasn’t until a week or so ago that I fully understood what I believe is the core problem behind Microsoft’s current ills.

Recently, while waiting to pay for my double espresso (black, no sugar) at a local coffee shop, I noticed that the store’s cash register had been replaced by…an iPad! I asked the salesperson how the iPad register was working out. She replied that it was working very well indeed.

A few days later, a problem with my home alarm system required a technician to come to my house. After finishing the repair, he whipped out an iPhone to log his work order and to communicate with his “home base.”

A week later, on a trip to Colorado, I took a door-to-door shuttle from the Denver Airport. Sitting in the front of the van, I noticed an Android tablet mounted on the dashboard. It contained the drop-off addresses for all the passengers, transmitted to the van from ticketing data stored elsewhere. The driver told me that, via the GPS hardware in the tablet, the central office could also track where he was located in real time.

Such smartphone and tablet sightings have become increasingly common. And that’s when the nickel dropped for me. This is exactly what is currently killing Microsoft.

It’s an understatement to say mobile devices have been a huge hit with consumers. Pretty soon, everyone who can afford a mobile phone will own a smartphone. It’s also true that Apple’s historical edge over Microsoft, if there has been one at all, has been in the home, education and creative professional markets. In contrast, in the business sector, it has always been Microsoft, Microsoft, and Microsoft. Sales to businesses, from small offices to large corporations to government institutions, have been Microsoft’s bread-and-butter. The have had no competitors.

Unfortunately, for Microsoft, the mobile device sightings cited above are all instances of smartphones and tablets being used in business environments. Businesses are increasingly using mobile devices for tasks that, a decade or more ago, would have been handled by a PC (desktop or laptop) running Windows. Instead, these tasks are now being handled by mobile devices via software written for iOS or Android devices.

That’s the core of Microsoft’s problem. It’s not merely declining PC sales — nor that Microsoft missed the boat entirely, in terms of aligning with compelling mobile products. It’s that, more than anything else, Microsoft’s success depends on the success of Windows. Until recently, Microsoft could breathe easy knowing that Windows was not only the dominant OS in business environments, it was just about the only OS. If a developer wanted to write a business application, they wrote it for Windows — and often nothing else.

This is now changing — and not in a good way for Microsoft. As tablets and smartphones invade the territory that used to be the exclusive domain of PCs, developers are increasingly writing business apps for these mobile devices, diminishing the role of Windows.

The situation is not yet so dire that it is past the “tipping point” for Microsoft. But the current trajectory for Microsoft looks increasingly bleak. Microsoft’s biggest threat is that “post-PC” ultimately translates as “post-Windows.” Microsoft’s main competitor is no longer the Mac, but mobile devices. Unless Microsoft can find a way to compete successfully with iOS and Android, Windows will become more and more marginalized in the years ahead. The day could yet come when Microsoft finds itself in a position not much different from where Blackberry is today.

Addendum: While initial drafts of this column were written days ago, Microsoft today announced the “retirement” of CEO Steve Ballmer. The timing of this posting is thus a coincidence. Still, the Ballmer news led to a flurry of articles on what ails Microsoft. Some, such as this one from Harry McCracken, wound up taking different routes to the same general conclusion as I expressed here.