You really want to check out Allegiance: http://www.freeallegiance.org/
I see that it's mentioned in the 14 games list.
Falcon 4.0 killed the combat flight sim market, more or less. Though it is still around, we don't have anywhere near the number of combat flight sim games that we used to. Non combat sims were pretty much always dominated by Microsoft, and now we have X-Plane to compete with it.
Xbox killed Mechwarrior, which was the premier sf vehicle sim that didn't involve spaceflight, though there's hope that it may return.
As far as the spaceflight sims go, it's hard to pin down an single point of abandon for developers. A combination of things, certainly, Xbox development poaching pc development, Descent, chnaging market tastes, etc. A more detailed study might be of interest. There's certainly still an active market for them, as weekly postings in the forum show, particularly if they're of the Elite trading+spaceflight style.