As best as I can tell, this whole post-PC era we're supposedly in is nonsense. Tablets, phones and other cute consumption devices are neat, and I wouldn't mind a tablet myself when they eventually mature, but there's no replacement for my home workstation. I've built my last few desktops myself and my current desktop is hitting around the 3 year mark and I'm starting to look at my upgrade path. I just got a new GPU, the CPU, RAM, mobo and PSU are my next upgrades and will likely occur as one single big hardware swap. An SSD would be nice too!
Back to the topic at hand: I don't see many people I know using their tablets to completely replace their 'real' computers. For some people laptops have started to replace desktops because they have lower demands and realistically laptop hardware seems to be much more on par than it was five years ago. As that continues to improve more people will probably ditch the desktop for a laptop, but that's still a 'PC' and there are still upgrade options like RAM and drives. I still wouldn't ditch a desktop for a laptop but in either scenario Newegg can continue to be successful. They sell laptops, they sell replacement parts.
Even if the Post-PC era weren't just marketing hype and news headline making nonsense, they still have plenty they can offer. NewEgg sells tablets too, they also sell software, home entertainment gear (I just got a new receiver from them) and all sorts of other things. I believe they have the ability to adjust themselves to changes in demand as needed, but I don't really think the PC business is in any danger of crumbling beneath them any time soon. New uses will emerge for computers, new games will come out demanding the latest technology and the best price/performance and the best choice for expansion continues to be the "desktop."