I think while it makes sense to play out the way it did it wasn't a satisfying conclusion. Cory was playing a losing hand like a typical megalomaniacal villain so it wasn't a desperate play on our part nor a look at a desperate man's world crumbling around him.
If we're talking about Mass Effect then I'd hope they play with different ways of building to a conclusion than just giving the villain a space magic twilight device. Not that it's easy to make (realistic) conflicts follow good narrative pacing.
Yeah, well, that's the trick, isn't it?
Probably the closest thing I can think of in terms of a conflict that did just that is the First World War. In March/April 1918, Germany came extremely close to winning the war (maybe even as close as a couple of miles, depending on how you feel about Clemenceau's postwar comments about Amiens). The tide turned decisively in July and August, and by September and October the Germans were in full retreat; the war wrapped up in November. That's a relatively rapid
dénouement in military terms, a testament to the Germans' enormous gamble in the Spring Offensives and the extremely heavy attrition the German army suffered in a short period of time. But it was still
seven to eight months between "Germany comes within an ace of victory" and "Germany defeated".
As you recognize, the fundamental calculus here is the problem: in order for the enemy to pose a credible threat all the way up to an apocalyptic finale, it has to be really strong, and it's simply not possible for most such enemies to be so thoroughly defeated in a single Bond-movie battle that their threat could just evaporate and be satisfactorily wiped out by the end of the story.
Of course, a lot of games and stories don't actually see that as an issue. Take the enormous narrative output on the Second World War. Where drama exists for Allied soldiers, it's not created by the imminent threat of a German/Japanese victory, unless the story ends in 1941 or 1942 with the war itself unresolved. Instead, the stories of the individual characters are what matter: do they survive, do they complete a specific mission, and so on and so forth. The war is a backdrop against which millions of individual human stories are played out - it's not, in itself, the narrative arc. And frankly, unless the focus is on a political leader, it wouldn't make any sense for the war to be the narrative arc! It's not like Private Ivanov is determining the fate of entire nations himself.
Since I never much liked the outsize impact that Shepard could have on galactic affairs, you can see why the notion of an entire war being a narrative arc for a game is something I find to be silly.