Ehh, I don't agree with the article's take on the subject, but I do agree with the title of the thread.
Reasons I believe the US gamer population is declining and thoughts about the article:
1. The economy is, and has been, in the tank. People have less money and are naturally less willing to spend it. This means that people save for a game they actually want rather than buying something that "might" be fun.
2. Game companies are charging for every little thing. DLC, passes, add-ons, to name a few. For example, I didn't buy MW3 for reasons related ( although not exclusive) to number one on my list. But my friends who did buy the game have been begging me to buy it now that the price has lowered ( not enough in my opinion). Thing is, if I bought the game now, it is reasonable to expect that I would also need to buy all the DLC that has come out for it. That takes the price from 35$ back to over 60$.
3. The article implies that people are switching to mobile device games and other similar "casual" styles. I don't believe they are switching. The most likely explanation is that people are bored, download a game to their phone, and play it until they aren't bored. After which, they forget they have the game. The game is still counted as active even if it's only been used for a few minutes. Additionally, people are downloading a majority of these games because they are FREE! Which makes a huge difference in how quickly a media style spreads. I would have never played Angry Birds, Fruit Slice, or Words with Friends if they weren't free.
4. Family games are in decline because people have started to realize that motion control is a gimmick.
5. Popularity. The US government, FDA, school system, and others have all been pushing for healthier lifestyles due to America's problem with obesity. Sitting in front of a screen for hours on end does nothing but add to the obesity problem. As a result, gaming and television have, and will, continue to see declines in their populations, as more Americans get off their ass and go outside for physical activity.
6. Finally, the article is a study of this year, which is not over and thus scews the results. Summer, spring, and the latter half of last years winter ( which was virtually nonexistant) have passed. Those seasons are the most popular times for outdoor activity and group activities, which naturally leads even more people away from the game screen. This fall and first half of winter should see increases across the board as people go back indoors and stay home more often. It will also see an increase due to high profile game releases, the holidays, and Christmas bonuses.
I predict that over the next several years we will continue to see declines in gaming, with small resurgances in the fall and winter. Ultimately though, I do believe that in five years or so, the gaming industry will be complaining about record lows year after year, while looking at increases in mobile games and wondering how they are being outpaced by the free 5 minute games that seem so popular.