While I agree that most serious people will continue to "roll their own", the fall of traditional desktops does have an impact on those who will always make their own computer as well as on traditional consumers. From a practical standpoint I imagine it will mean more expensive components as fewer people buy them. It could also mean less competition as manufacturers leave the desktop market for mobile computing, and with less competition less innovation. Already most important new technologies in displays like OLED are almost entirely found in mobile platforms. It will be interesting to see when graphene or photonic or any other next generation chips begin to become available where they appear first, in desktops or in mobile.

However, honestly, I'm not too worried about the practical hardware considerations, desktops will always have at the very least a market among gamers (like you pointed out). I'm more concerned about the general trends towards convenience and simplicity over freedom and deep content creation of which mobile computing is both a symptom and promoter. Cellphones especially, but mobile computing in general, wants to present information as quickly and as succinctly as possible. Consumers have bought into this as instant gratification and convenience are very nice. But we have bought it at a price, at least thus far, by getting devices we intend to replace every year or two, by getting locked into disparate ecosystems, by getting sound bites instead of a reality of complexity. There are certainly many apps and new OS's that try to combat this, but I can't shake the feeling that a society on mobile technology maybe sees more, but grasps less. This is all very cynical of me of course, and being a PC gamer makes me (probably very) biased.