Windows was, at first, designed to run on multiple hardware platforms. The objective was an "operating environment" that would give Microsoft an opportunity to sell software for multiple hardware platforms. There were versions of Windows for the Zenith Z-100 and the DEC Rainbow; these computers had Intel processors and ran MS-DOS but used architectures different from the IBM PC. Later versions of Windows ran on PowerPC, DEC Alpha, and MIPS processors. Those variants have all ceased; Microsoft supports only Intel PC architecture for "real" Windows and the new Windows RT variant for ARM processors, and both of these run on well-defined hardware.
The .NET platform has also narrowed. Instead of machine architectures, the narrowing has been with programming languages. When Microsoft released .NET, it supplied compilers for four languages: C++, C#, Visual Basic, and Visual J#. Microsoft also made bold proclamations about the .NET platform supporting multiple languages; the implications were that other vendors would build compilers and that Java was a "one-trick pony".
Yet the broad support for languages has narrowed. It was clear from the start that Microsoft was supporting C# as "first among equals". The documentation for C# was more complete and better organized than the documentation for other languages. Other vendors did provide compilers for other languages (and some still do), but the .NET world is pretty much C# with a small set of VB fans. Microsoft's forays into Python and Ruby (the IronPython and IronRuby engines) have been spun off as separate projects; the only "expansion" language from Microsoft is F#, used for functional programming.
Another word for this narrowing of technology is "specialization". Microsoft focussed Windows on the PC platform; the code become specialized. The .NET ecosystem is narrowing to C#; our code is becoming specialized.
Specialization has its advantages. Limiting Windows to the PC architecture reduced Microsoft's costs and enabled them to optimize Windows for the platform. (Later, Microsoft would become strong enough to specify the hardware platform, and they made sure that advances in PC hardware meshed with improvements in Windows.)
Yet specialization is not without risk. When one is optimized for an environment (such as PC hardware or a language), it is hard to move to another environment. Thus, Windows is a great operating system for desktop PCs but a poor fit on tablets. Windows 8 shows that significant changes are needed to move to tablets.
Similarly, specializing in C# may lead to significant hurdles when new programming paradigms emerge. The .NET platform is optimized for C# and its object-oriented roots. Moving to another programming paradigm (such as functional programming) may prove difficult. The IronPython and IronRuby projects may provide some leverage, as may the F# language, but these are quite small compared to C# in the .NET ecosystem.
Interestingly, the "one-trick pony" environment for Java has expanded to include Clojure, Groovy, and Scala, as well as Jython and JRuby. So not all technologies narrow, and
Picking the target for your technology is a delicate balance. A broad set of targets leads to performance issues and markets with little return. A narrow set of targets reduces costs but foregoes market penetration (and revenue) and may leave you ill-prepared for a paradigm shift. You have to chart your way between the two.
I didn't say it would be easy.
