I think Desktops in large and medium size enterprises are (or will be) facing strong competition from thin clients running virtual desktops. Thin clients such as
http://www.samsung.com/us/business/m...LF19PPBCBNA/ZA . These offer the advantage that not only are they easier (and therefore cheaper) to maintain than a fleet of PCs, but they are easier to secure.
As for your comment about desktops being easier to upgrade. They are, but bear in mind that most home or commercial users will not bother with that. Also, with the adoption of technologies such as USB 3 and Thunderbolt, laptops and thin clients may well end up almost as expandable as Desktops.