A good point, but then again if you couldn't defeat the big bad guy at the end of the game, you'd feel a little bit cheated, wouldn't you. Anyone remember the old Quake conversion Zerstorer? hur hur. Closure at the end of a game is most necessary, but because of that the medium doesn't lend itself to contiguous antagonism. How many game villains have been ressurected from certain death time and time again? Kane from C&C... Shodan in SS2... Shang Tsung in Mortal Kombat... Doh in Arkanoid, the list is endless. Games are episodic in nature and until someone releases a truly realised series of games with a consecutive and believable plotline, they won't be anything but. And such an undertaking would be a large affair. A project comparable to the LotR movie series, and requiring a similar leap of faith by financial backers.
Heavy Metal: Fakk 2 seems have been set up that way. The game ends about half way thru the story. Too bad it didn't sell very well. It was a cool game.