Post-PC doesn't mean there will be no more PC's, it just means that consumers are going to shift their primary usage to devices like tablets & smartphones, and many won't bother to even own a PC. It does not mean that PC's will go away nor does it mean that smartphones will become the dominant development stack.
The overall PC market in 2011 has been shrinking, while tablet and smartphone sales are growing rapidly. This trend will have to last longer than a year to be strong evidence of anything, but it does hint that the Post-PC notion might not just be "marketing hype".
For people who do not understand their PCs (most consumers), the simplicity of a tablet is very attractive. After all, functionality that you are afraid to use or avoid because it is too complicated might as well not exist. (And no, all of these people who haven't figured out a PC in the last 20 years are not going to en-masse decide figure it all out.) Anyone who has built their own PC, uses 2+ monitors, professionally requires lots of computing power or is a gamer is not going to give up their PC in a Post-PC world. (So NewEgg will be just fine, as these are their primary customers)
Nobody is arguing that professionals will give up their workstations, or even that that would make any sense. You need a PC to write iOS applications, and I would be very surprised if that changed anytime soon.