Those of you familiar with XCOM: Enemy Unknown have realised that I ended my narrative before the game itself ends. If you really want to know how the game as shipped ends, you should take a gander at Jarenth’s version of the tale. I really hate the game’s canon ending, and I really wasn’t looking forward to guiding McNutcase’s team through it. The end is thoroughly unsatisfying – it’s blatantly obvious sequel-bait. When I managed to accidentally write a far more satisfying ending after completing two missions in a single day, I could not pass up the opportunity.
In the original X-COM, the conflict escalates along a quasi-parallel course, but it diverges after raiding the alien base. In Enemy Unknown, the alien base raid is followed by having to track down an overseer ship which is designed to only really be defeated by the Firestorm, a reverse-engineered fighter version of the aliens’ own craft. The Commander’s lamentations about air superiority were meant to be foreshadowing to the development of the Firestorm, which drastically changes the odds in the skies. Once the overseer ship is captured, the endgame with the Temple ship is triggered, though I am unaware of any true deadline on having to defeat it.
In UFO Defence (aka, UFO: Enemy Unknown, the original X-COM), the alien base assault leads to discovering the location of the alien’s true headquarters – on Mars. At that point, the aliens are no longer the aggressors, but are instead on the defensive. The next move is X-COM’s, who launch an assault on the Mars base, invading it to locate and kill the alien brain – literally a brain, as fitting the time – that controls the alien invasion.
The newer XCOM does not give a clear resolution to the story; all that happens is Earth is not destroyed. The aliens are not defeated and there is no sign that the war has actually come to a conclusion. In the older X-COM, however, the war is decisively won by humanity, with the alien threat completely neutralised. In short, it actually ends, and because it does, it gives a far more satisfying finale.
Since I was going to be stuck with an open-ended finale either way, I chose one that was more a continuing conflict than some lame “noble” sacrifice to defuse the Deus Ex Machina doomsday device suddenly introduced nine-tenths through the story.