I don't think Desktops will just disappear....
The hardware in Desktops will ALWAYS be superior for 2 reasons:
1. More room for better and bigger hardware.
2. More room for better and efficient cooling.
With a high end gaming laptop, you would be lucky to get a steady 25 FPS. I would love to see someone try to play Crysis 3 on a tablet. Even a really high end gaming laptop will most likely be brought to its knees on high settings. In fact, I figure two 7970's in CrossfireX or an Nvidia SLI equavilent will be required to run that game on all settings maxed at playable frame rates @1080p
Yes there is a difference between 30 fps and 60 fps to the human eye. I can notice it, and im sure other PC gamers notice it to. Console graphics look like garbage after playing the same game on a real PC.
As for cooling....compiling huge amounts of code that peg a 4 core, even an 8 core CPU at 100% for extended periods of time are going to cause tablets, and some laptops, some serious issues with heat. These systems were not designed for that kind of heavy lifting.
Will mobile device get more popular? sure they will. Will they be the right tool in every instance? no. Will PC go away? not a chance.
in fact, I bet online part retailers like Newegg and others won't miss a beat...in fact they may gain even more business. Steam sales i do beleive are at an all time high and they cater to the PC gaming crowd. PC's are far from dead folks