One of the main reasons that Windows would not be suitable for a Keyboard O/S is that Windows was never designed as a "real-time" operating system. When playing a keybaord the system has to respond almost instantly to events especially key-presses and changes in instrument patches etc. Anyone who has used a PC with Windows will tell you of the times it needs to go away and think about things and this would be completely unacceptable in a playing situation. The O/S on Technics Yamaha etc may have their failings, but at least they are optimised for the instrument responding within a milli-second or 2 to key press actions by the player. Other dedicated computers, such as the engine management system on your car have to be designed in the same way and I dont think anyone is suggesting that Windows is used as the O/S for car fuel systems.

Windows has achieved its pre-eminence as a file management system, not as a real-time control system.