I can tell you what's killing PC gaming: graphics.
People expect every new game to look 10x better than its predecessor, or else you see these posts like "it looks like ass" or "so 2005!" or "consolized!" So they get bigger budgets devoted almost solely to the graphics over the gameplay. We get (aside from RPGs) shorter games with less content over more FLASH. There's no resources for gameplay and content. Then the game won't run right on a PC that isn't high-end or it has to be detuned so it looks poor. Doom 3 was excellent at running well on low-end machines without sacrificing too much quality, and its sales showed. Of course, people complained about the gameplay and content...and the graphics!! So people don't buy games unless they have great next-gen graphics AND great gameplay AND lots of content (CoD4 proved the rule by being the exception) AND run great on older machines...so the PC games lose money even with good sales because anything under 1 million sold isn't worth continuing with! Unless of course you just make a port from a better-selling console version that WILL run well looking like it does on the box.