Has another Star Wars tabletop game been around very long? I can't think of any, except maybe the WEG roleplaying game. For various reasons, I don't see X-Wing as a game that can survive very long without new products being published.
However, I also do not agree that the reservoir of possible expansions will be empty soon. There are four more films to draw on as well as the Rebels cartoons. Reprints in the vein of 'aces' or 'veterans' packs might become a bit more common, but there is also the possibility of an expansion branching out in some kind of new territory, like space stations, ground combat, scenario/cinematic expansions, or something we haven't really thought about. All this is assuming the prequels are left untouched.