The key point is the PC gaming landscape have ceased to be the domain of retail games. Its being fragmented into subscription-based, ad-based, downloadable, etc. The PC gaming alliance is doing right by changing the method of tracking the revenue. Retail is now one of the many subsections of PC gaming sales channel hence the old way of tracking the industry is obsolete.

As for the health of the PC gaming industry, factors such as more leisure activities on the PC(youtube, facebook, etc), and external ones such as new consoles, handheld and the Iphone, all of these decreases the market size of PC gaming. Its all part of market dynamics. But after all of this change is done, PC gaming will be leaner and meaner. The graphical arms race will stop, more indie games on the pc, more social games (VW, MMO), better browser games.... All of these will grant the PC its truly unique identity compared to consoles, handheld and mobile gaming.