Go try to run FS98 for more than a few hours.
In 2015 FSX will REALLY start to show its age. By 2015 if we arent using quantum computing cards we will be using cards that are able to render 2x what they can now. Nvidia is already getting ready to get a new hi end card out that has an insane amount of computing ability.
While I think FSX has 5 years. I got to admit now that I think about it. That is about it. At that point it will be better modify a combat sim than continue to mess with FSX.
MS is betting (I suppose) that with the downturn of the economy, fewer people are going to be looking to buying high-end rigs. With fewer demand for high-end computing, there will be less advancement at the high end of the spectrum. MSFS games have always been written to do well on computers that don't exist yet (look at FSX, systems that can run it really well have only been around for a few months), and I guess they decided that there was no more room for advancement if the computers to run the thing weren't going to be there.
Nvidia just released the GTX295 and 285; I doubt that there will be any new cards coming from them anytime soon (plus, they're out of numbers in the 2xx series

). The 3xx series isn't due until Q3-Q4 2009, from what I've heard.
It wouldn't surprise me at all if we do see a lessening of the advancement curve at the high-end of the computing spectrum over the next few months/years. With money starting to be tight, there just won't be as many buyers for the things.
As for games getting old: Warcraft III was released nearly seven years ago; Frozen Throne nearly six. There's still a massive number of players for both (mostly due to DotA, probably) online. How many people still play Starcraft, and how old is that?
Games that sit at the pinnacle of their genre tend to stick around for a long time. I doubt that any flight sim is going to come close to FSX anytime soon, and as the current high-end that's needed to run it beautifully becomes cheaper, more people will go to it instead of FS2004.