I've tried using a touchscreen desktop machine at work. It was an unnatural and uncomfortable experience. I ended up sticking a proper keyboard and mouse on it!
Virtually all of these machine the screen is near vertical, which means at a proper viewing distance, to operate things on the screen using touch, requires your arm to be lifted and outstretched. Try doing that for more than a few minutes continuously. The only time we found it made sense, was when we created a bespoke console, with a near horizontal touchscreen. That was more comfortable to use.
Also large displays require large hand movements to achieve most of the things you do with a few centimetres of movement with a mouse. My personal view is that about 10 to 11 inches, touch screens start to become less effective.