The PC is an extremely refined instrument with excellent flexibility. Billions of hours have been spent training users in how to type and click with a mouse/trackpad/trackball and navigate operating systems. It will take something vastly superior to replace them.

With respect to productivity and usefulness, touch screen devices are mostly inferior, aside from being very mobile. They have little to no tactile feedback, are laggy, and controls can change even as you are inputting them. I've very often tried to click a button/hyperlink on a touch screen and had it change at the last half second.