While I believe the retail
PC market is fading away, the digital
PC scene is flourishing. Have you ever heard of Steam? It's a digital marketplace, akin to Xbox Live Marketplace/PSN/Wii Shop, with a rather large selection of games to buy from a large spectrum of game companies and developers. It also has community features similar to consoles.
I personally belive that FPS for the CPU will no longer exist with the Wii and other consoles around
First person games (whether they are shooters or adventure games) on the PC are actually extremely popular, and have been for years (Counter-Strike, Battlefield Series, Team Fortress 2, etc). The reason is that a mouse allows for superior aiming and control VS a controller analog stick, or even the Wii. While many PC games are also available on consoles, PC versions most likely offer higher player count support, user-created servers, etc.
On top of that, there is the mod community. Mods allow players to customize, sometimes to insane extents, numerous aspects of video games. Some game developers publish their games with the mod community in mind (Valve), and this creates a unique user-created-content mixing with developer-created-content relationship. It is possible to mod console games, but it is usually difficult and you end up usually using the PC anyway. Modding on the PC can be as easy as downloading a program someone else made or simply entering a few line of text into a command console.
And of course, as you mentioned, certain niche genres really shine their best on the PC. Most RTS games suffer in translation when put on a console; not having a mouse to navigate severely hampers gameplay, so usually the game is just not the same. MMOs have been on both console and PC, but unless the community is shared (i.e. players from both platforms are on the same servers), usually the console MMOs tend to die out or remain lowly populated.
While I am not a big PC game player per se (don't have that great of computer for gaming), I do believe PC gaming is still thriving, albeit under our noses.