The last good FPS I've played was S.T.A.L.K.E.R.: Shadow of Chernobyl, and in my humble opinion the only way any FPS game can succeed especially nowadays is to be PC-exclusive AND to have an official editor for the community to temper with and create tons of modifications. That's what PC gaming is about, giving and creating options to and for the players, so that you can play the game "as is" (vanilla, unmodified), and then explore new possibilities later on to keep the replay value virtually infinite.
And I'm not speaking about a measly editing tool to just be able to create new textures, I'm talking about something on the level of Bethesda's editor (possibilities-wise), in which talented/imaginative people can (if they can indeed) literally create their own expansion pack or combine multiple modifications into one. Give me FPS games like that, FPS games that can be modified and are PC-exclusive, and suddenly I will start "forgetting" about the seemingly nostalgia-driven desires to play FPS games that were released anywhere between five to ten years ago. In the meantime I'm still happy launching either the original DOOM or Quake, Duke Nukem 3D, or Tribes or Unreal Tournament '99.
The "most recent" FPS games I've bought were Battlefield 2 and Bad Company 2, and I guess I can include Prey and Quake IV in there as well as being "recent" (of course the definition of what is recent is subjective here), amongst a few others, and even though I liked them for a time I just don't have the desire to come back regularly to play them again, but I have absolutely no difficulty whatsoever in launching FPS games that are literally a decade older on a regular basis, even if they weren't (nor aren't still today) modifiable back then, heck I even have the original Halo installed and I play that one more often than any of my more recent FPS'es, and it's a console port for crying out loud.
I just haven't played any really good FPS since SoC and that's a almost four years-old by now (and yes I liked Clear Sky and CoP but I still prefer SoC). The quality of FPS games over time since the past decade or so has indeed decreased dramatically, in my opinion, and only a very small number of exceptions stand out, every now and then.