There's also the fact the FSO already has a wealth of material available for it in the form of Mods and Upgrades. As Will Wright once said, something that will keep a fan-base interested in a game for a long period of time is the ability to create their own content, so whilst other games may flare into existence, the obsession with protecting IP in many companies means that flexibility is limited, also the tools available to console oriented games tend to also be limited.
This gives FSO a longevity that will be difficult to match from a company perspective, so whilst it may be outshined from time to time, after that game has faded, FSO will still be there, evolving and changing to suit the times.