The high-end PC gaming market will, I'm sure, keep the desktop / tower form factor going for quite some time yet. That bleeding edge will always be populated by insanely powerful devices requiring bulky heatsinks, fans and power supplies. For normal use, yes, things are moving in the "pocket computer" direction, but no matter how powerful these mobile CPUs and GPUs get, you'll always be able to make them go faster by increasing the clock speed / number of cores / number of buses and adding a massive heatsink. And I'm sure there'll be a market for that for a long time.