I see a parallel to OS development in commercial airplanes.
If you look at a picture of an airplane from the first 50 years of aviation, you can usually guess with 10 years when it was first built.
There was tremendous development going from wooden airframes covered with canvas to metal airframes, streamlining to jet engines, and then even though there have still been improvements, the look of airplanes hasn't changed much in 50 years.
There are still 707s and DC-8s in service, heck the average B-52 is around 50 years old.
The point is you get to a point with a technology where the technology matures and there isn't a big demand for the next step.
The Concorde is now retired from service and the supersonic look never took off with commercial aviation.
At some point in time the OS will be fine, the processing speed will be fine and there just won't be the new applications that make your computer so slow you need to get a new one.
Microsoft monopolized the OS market when there weren't any Operating Systems in use, so they have dominated through a period of market expansion, but once the market gets saturated where does the revenue come from?

Each upgrade cycle is taking longer, I was replacing windows 3.1 machines with XP machines last year, those new machines could be around for 15-20 years, there just isn't the need to upgrade the technology to meet business requirements.

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