I can see two things that currently set PC gaming apart from both consoles and mobile devices (for the most part), and I doubt that they're going anywhere fast.
1. PC gaming is an open platform. I notice you didn't even make mention of the indie development scene - an omission which can be charitably described as "unfortunate." Anyone can make a game on the PC - heck, I've made about a dozen myself. You don't need to meet arbitrary quality standards, have a development team in the hundreds and a publisher, or pay the platform holder a weighty Danegeld for the "privilege" to coexist on their platform. For all the inane boasting about Xbox Live and PSN, they still have less real innovation, less novelty and fewer quality games on them (that MS and Sony didn't buy, that is) than the PC does. Likewise, the combination of the digital distribution revolution and the middle-market (game developers who focus on niche categories with small teams and low development costs) is bringing to light an entirely new population of talented people, rushing to fill the void left by the profit-hungry publishers and their overfed, unwieldy pet development teams. Yes, the casual market is big and getting bigger, but we of the "hardcore" are not going anywhere.
2. PC gaming is a constantly advancing field. Where consoles, tablets, and mobile architecture are fire-and-forget hardware, launched slightly ahead of the curve every five or seven years, then left to become hopelessly outdated as time goes on, PCs have the luxury of getting better and better over time. This means that developer missteps, poor optimization, and expensive hardware costs are eventually overcome by the passage of time, and the PC gamers who don't have the latest, greatest hardware now can play your game on very high settings at insane resolutions sometime down the road. This is a mixed blessing - it means you have to keep upgrading your hardware as time goes on, if you want to keep up with the system - but the potential benefits are plain to see, as Crysis surpassed the consoles' graphical ceiling 3.5 years ago, and nowadays even an RTS can look better than most console games.
Yes, the PC hardware setup may become irrelevant. Tablets and laptops and other fire-and-forget devices may overtake it completely. But PC gaming will not die, sir, not even in its most traditional form. It will simply move on, with its loyal followers, its independent developers, and its niche audiences, into a new and fertile land.