Here's another example if you're a Doctor Who fan: the classic cliffhanger to part 1 of The Caves of Androzani (spoilers ahead). It would be like saying the show is ruined because we just saw the Doctor and companion executed by firing squad. Yes the scene happened, but no they're not actually dead. The following week we learn that they had been switched out by android dummies.
I'm not saying that happens on B5... But the events of the next few years does
change the meaning of War Without End. Kirk, for your own enjoyment, try not to dismiss future Londo scenes by thinking, "well none of this really matters because I already know how he ends up".