Desktops aren't 'taking a hit from tablets', they're 'taking a hit from mobile platforms'. Phones, laptops, and tablets have all increased. Desktops have decreased. Tablets are chewing out part of the laptop market, but laptops in turn are chewing out part of the desktop market. Consider the desktop to laptop ratio you saw in offices 5 years ago compared to now.

I'm not saying desktops are dying, just that they're no longer 'the default' they once were.