Desktops are never going to be mainly controlled with a touchscreen.
Because laser mice are 5700 dpi (and improving), while monitors are 96 dpi: hence, you need to move your arm a distance which is *50 TIMES* larger when using a touchscreen, and also keep your arm in a very uncomfortable position.
Also, with a physical keyboard you can rest your hands on it without pressing the keys, while you can't do that with a virtual keyboard.
Mouse and keyboard will only be replaced when it is possible to directly read the user's brain.
If you don't believe this, just try it yourself, by pretending that your desktop monitor is a touchscreen even if isn't, and trying using it as such for a few minutes.