Sure these devices are becoming more popular. But how many people go in to their office and sat at an iPad, or Android phone to work all day long. As far as comfortable way top use a computer all day long, the desktop is still way ahead of these touchscreen devices and will be for a while yet.
Yes, I think it was Steve Jobs comparing iPad to desktops saying "desktops are like trucks", meaning you use them for the heavy lifting but not for anything fun. But this may change when productivity software catches the Touch paradigm. Remember when desktops were just for hobbyist use and the real work happened on terminal applications accessing mainframes?