It's easy to think of the history of Windows in purely commercial terms, with each new version serving as a way to pour revenue into Microsoft's coffers. But there's another way to look at those releases as well, by focusing on the major technologies that debuted in each one, even when they weren't, strictly speaking, ready for primetime.
In fact, focusing on those technologies might help explain the familiar cycle of Windows releases, where early versions are problematic and it takes at least two major updates before things settle down.
Looking back at this history helps explain why some features that seemed baffling at the time eventually became key to future Windows releases. It also explains a few spectacular failures.
1993-2000: Windows NT upgrades to business class
Windows NT (short for "New Technology") was Microsoft's first honest-to-goodness business operating system, with a preemptive multitasking kernel and a journaled, secure file system. It was a stark contrast to the glorified MS-DOS shell that was the consumer version of Windows 3.1 for the masses. The first NT release, in 1993, was startling for its system requirements: the Workstation version required 12 MB of RAM and 90 MB of free hard disk space. And no, those are not typos. The Windows NT family lives on to this day in some of the core components of Windows 10.
1995: Windows 95 goes 32-bit
The spectacle of Windows 95 is all that anyone remembers today. Microsoft paid a fortune to license the Rolling Stones' "Start Me Up," a tribute to the new Start button and the Windows 95 desktop. But the bigger technical driver for this world-changing upgrade was the desire to replace MS-DOS once and for all. A few bits of 16-bit code were still buried in the original release of Windows 95, but those pieces vanished by the next release, Windows 98.
2001: Windows XP consolidates two OS families
This release marked the merger of Windows' home and business versions in a single product, with the NT line officially prevailing over the Windows 9x alternative. Although it was a major milestone commercially, the timing could not have been worse. The Windows XP launch event was held in New York City in October 2001, just weeks after the 9/11 attacks, and the new OS was immediately beset by security issues that took several years to resolve.
2003-2017: Windows Phone/Mobile
Over the course of more than a decade, Microsoft introduced multiple variations of Windows on mobile devices. The name kept changing: first the stylus-centric Windows Mobile, then Windows Phone, and finally Windows 10 Mobile. The developer toolsets kept changing as well, which created an endless source of frustration for app makers. Microsoft finally pulled the plug on Windows 10 Mobile in 2017, and today its mobile efforts are focused on apps and services for the two leading mobile operating systems, iOS and Android.
2012: Windows 8 rolls snake eyes
"We've reimagined Windows and we've reimagined the whole PC industry," CEO Steve Ballmer told Reuters ahead of a launch event for Windows 8 in New York City. "In addition to notebooks and desktops, we introduce the PC as tablet," said Ballmer, in his characteristic bluster. "Work. Play. Tablet. PC. Boom! One product."
It was a bold response to the success of Apple's iPad, but the big bet didn't pay off. Two major technical changes have survived, however: the Microsoft app store and ubiquitous touchscreen support.
2012: Windows RT tries and fails to break the Wintel duopoly
After a brief flirtation with RISC processors in the early days of Windows NT, Windows had settled into a long marriage with Intel's x86 chip architecture. Slapping the brand-new Windows 8 interface on an operating system with the Arm processor was another bold move that also turned out to be a money-losing short-term bet. Windows RT had only a brief life, but the work Microsoft did to port Windows to the Arm platform is finally paying off seven years later.
2015: Windows 10 undoes the damage of Windows 8
From a commercial point of view, Windows 10 had one job, which was to erase the unpleasant memory of Windows 8. It accomplished that mission by undoing much of the user experience that had made Windows 8 so disorienting and adopting a more evolutionary Windows desktop. But the bigger change in Windows 10 was the debut of what the company has dubbed "Windows as a Service." That's a high-falutin' name for a series of changes to the underpinnings of Windows Update, focusing on cumulative monthly security patches and free semi-annual feature updates.
2017: Windows 10 S locks itself out of its target market
This might be the single most baffling Windows version ever released. It was so confusing, in fact, that I wound up writing multiple articles to try to explain What Windows 10 S was supposed to be and then to decipher what it could and couldn't do. (The short version: You're limited to apps from the Microsoft Store, and you can't use many administrative tools.) Microsoft didn't help by launching it on the Surface Laptop and positioning the new Windows edition for the education market. Within a year, Microsoft acknowledged the obvious and repositioned Windows 10 S as a feature rather than a separate Windows edition.
2019: Windows Virtual Desktop moves to the cloud
When you say "Windows as a service," this is what you mean. Windows Virtual Desktop, which was released for commercial customers in late 2019 after a long beta period, is Windows 10, running in Microsoft's Azure cloud as a fully managed service. Its major selling point is that it moves management and security challenges off the local PC and onto Microsoft's cloud. Perhaps this technology will migrate to consumer PCs someday, but for now it's purely an enterprise play.
2019: Windows 10X brings containers to the mainstream
The last time Microsoft tried to create a new edition by tacking a capital letter to the end of the Windows 10 name, it didn't work out well. Will this time be different? Windows 10X is debuting as the native operating system for a new PC form factor, with multiple screens. But its core innovation is the ability to run traditional Windows desktop apps in secure containers that are isolated from the core of the operating system. It's easy to imagine this technology migrating to more traditional form factors before long.