There's still a bit of life left in the Microsoft flight sim line yet, MS is apparently still going to publish and market FSX for the time being.
www.flightsim.com and
www.avsim.com are good repositories for aircraft, scenery and other add-ons for Flight Simulator 98 to FSX, they even have files for X-Plane and others.
Multiplayer will also keep it going for a few years to come. The built in multiplayer functionality found in FS98 - FS2004 is pretty cumbersome, FSX is the only version with pretty user friendly multiplayer and built in voice communications. There are alternatives though.
VATSIM has been around for years and supplies real time ATC via dedicated people running different airspaces, there are usually two or three that will control at least the east coast of Australia. For the more casual flightsimmers there is the recently started
Bigpond GameArena flighsim server which has no stringent rules or ATC requirements. Both of these options require the download of a couple of extra, but fairly easy to use programs. The biggest plus of these multiplayer services is that you can play no matter what version of flight simulator you have, handy if a friend has FS2004 and you have FSX. If your ISP is telstra, the gamearena server has the added bonus of being usage free, meaning it won't count towards your monthly download limit.