Steam is good content delivery system but will it be able to compete with XBOX Live, i really don't think so. Future for gaming is consoles, most of the in-game advertising will be done by the platform's like Microsoft, Sony and Nintendo.
Games are getting harder and more expensive to make, but projects like Irrlicht and Ogre 3d are becoming more complex. As the years go on the paradigm will change from a content centric design in PC games, to a complexity and community centric design philosophy. I can see a time when the community builds the environment within the bounds of the rules of the game engine - and I'm not talking about second life.
And where do you suggest all the millions of Modders will go?
Consoles are becoming computers, and I can guarantee the operating systems will look more and more like a simplified version of windows, or OSX as the years go on.... OpenGL, DirectX will continue to be developed. The simple fact alone that Console games must be developed on a PC is going to keep the PC kicking - it has to run emulated versions of the software that the console (lesser hardware) runs - PC Gaming isn't dead at all ... All you need to realize that is an internet connection and a POGO account. I realize this isn't the market segment you are talking about but the largest private website in the world is an online PC games site (http://en.wikipedia.org/wiki/Miniclip) As long as there is a pc - AND THERE always will be a device that allows for open development, and deployment of software - there will be games (irregardless of how complex) for that platform.
Bottom Line: as long as people have computers they, and their kids are going to want to put games on them.
Hey! Like Spore! I think it's a damn great idea :)