The biggest problem of PCs is the fast evolution of hardware. When you buy a game for PS3, you know it will work well on your PS3. And when you buy a PS3, you know you'll keep finding games for it, even in 3 years from now. And I bet there are still titles released for PS2. That's the only advantage of console gaming and is quite a big one.
The fast evolution of mobile devices is actually a disadvantage for the kind of people that buy gaming consoles in the first place.
However, console gaming has been a niche for quite some time. This is not new. The exception was Nintendo's Wii, but that was just a fluke.
http://www.gamasutra.com/view/news/31995/Exclusive_Black_Ops...
http://boardgames.about.com/od/monopolyfaq/f/copies_sold.htm
According to Hasbro, as of early 2008, more than 250 million copies of Monopoly have been sold worldwide. (As of early 2003, the estimate was "more than 200 million copies.") The company estimates that nearly 500 million people have played Monopoly.
Anyone want to guess how many decks of playing cards, or chess sets have been sold?
Digital distribution dominates on the PC now, and if your figures don't take it into account, then your figures aren't very useful.
On the other side everyone expects Apple to make something interesting with their Apple TV like enabling Apps or selling their own TV. I am not sure they will do because Airplay works fine. But a standalone box has a few advantages to mobile devices which only stream to the TV: More internal room. Unlimited electricity. Better Controllers.
@Cloud Gaming This is really interesting and has potential for disruption. But OnLive is not available in Europe, so I can't try out how good/bad it is working. I can only speculate lag will always bother the die-hard fans?
People have limited money to spend on electronics, and as gaming on consoles becomes better, more people will play games on them instead of buying dedicated gaming machines. With technology like AirPlay, consumers can play tablet games on big TVs.
The same hardware that is in tablets makes its way to phones, which is going to make portable gaming systems very niche. We may even see phones next year that can create better looking games than the XBOX 360 and PS3.
Cloud gaming is on the way, and this will further erode consoles. You'll be able to have a dumb terminal of a TV or set top box that uses servers to render gaming content. Better looking games will naturally happen as servers become cheaper and more powerful.
None of this will replace the console gaming experience for those that really want that experience, but for the more casual audience, it will begin to erode it. With tablets, AirPlay, cloud gaming, etc. what future does a console like the Wii have? Phones can even be used as motion controllers with all of their sensors built in.
There will be a future for XBOX 360 and PS3 style consoles, but I agree that it will be more niche and that the next generation will last a long time. Because of this, I hope the next generation of consoles are built to last a long time.