These figures are extrapolated from W3Schools.com. I have predicted data for the next 4 years with a linear scale based on the last 5 years. The vertical axis has a logarithmic scale so the OSX and Linux data is meaningful. The graph shows a very slight decline in Windows usage and a notable increase in Apple OSX and Linux usage.
Obviously there are plenty of unforseen changes that could have an effect on this graph. I see the move towards OSX from Windows in the mainstream as inevitable, and I would imagine the change is more likely to be a sudden surge over a period of months rather than a smooth decline of Windows usage over a number of years.
It is interesting to compare the initial Vista predictions made by Steven York just before Vista’s release with the real stats in hindsight. The obviously low take up of Vista leaves most users on Windows XP, which when you compare to the look and feel of modern environments (for example: Websites, Mobile Phones, Latest Apple OS) it’s very clunky and unintuitive.