Let's face it: very few hit films end with the credits scrolling over dead puppies and weeping children. Movie audiences pretty much demand happy endings.
But sometimes Hollywood slips one past us, giving us a supposed happy ending that is actually depressing as hell once you give it a little thought. For instance:
6Return of the Jedi
The "Happy" Ending:
The evil Emperor Palpatine hatches a plan to defeat the Rebellion once and for all by telling them the location of his unfinished superweapon and then telling them how to defeat it. This plan goes about exactly as well as you would expect, and our heroes destroy the evil scourge with the help of some genocidal teddy bears.
Wait a Minute...
That epic battle at the end there? That only destroyed one base and a fraction of the troops the Empire had at its disposal. Sure, Vader and the Emperor were both blown up, but that wouldn't destroy the Empire any more than blowing up the Pentagon would dissolve the USA.
What it does do is create a horrific power vacuum, in an empire with fleets of Star Destroyers and millions of pissed-off troops roving around the galaxy (even more pissed off if the payroll office and all of their checks were in the Death Star).
"So... should we just keep being Stormtroopers, or what?"
Soon these power-hungry military officers would no doubt form factions and destroy entire planets in their brutal attempts to seize power. Eventually Palpatine would simply be replaced by a new Emperor, possibly even one competent enough to devise a plan that can't be foiled by developmentally stunted bears throwing rocks.
Sure, the Death Star was taken out, but that didn't exactly stop them last time. Not to mention that by the time they reach phase twenty-six of their patented "Let's just throw Death Stars at the problem until it goes away" strategy, [below] someone's probably going to decide that maybe the ship doesn't really need an unguarded, torpedo-shaped hole on the outside, thus allowing the Empire to swiftly conquer the entire universe.