The point is that if EITHER evil OR good completely wins (with no hope of a shift later on) then you have a static situation, and static situations don't make for good storytelling. It is the struggle and tension between the two that makes for suspense and uncertainty. No one really wants to hear about what exactly "happily ever after" in a fairy tale involves because it would be boring. The words are a way of letting people know that the struggle is over and a static situation has ensued.

Theoretically, if evil won, it would be similar, only it would sound something like this:

And darkness covered the land, and Sauron destroyed all that was green and good in Middle Earth, and he caused all growing things to wither. When the orcs could find no other food, they fed upon each other and died off, until all that was left were wraiths and spiteful spirits in a wasteland of endless gloom.

The point is that, short of that sort of ending, there is ALWAYS an opportunity for movement and change. Think of the typical Hollywood ending - the villain crawls out the rubble after everyone thought he was dead, to plot a new dastardly scheme, or heroes crawl out of a blasted city and say, "We can rebuild this."

Interestingly enough, movies that aren't looking to make sequels often have endings where evil wins - Cloverfield comes to mind immediately, but there are plenty of others.