Say, I wonder, how come they would fight for eternity when they basically have angel swords that can kill even Archangels (as we've seen in Hammer of the Gods ). I mean, isn't one of them supposed to kill the other easily, regardless if it's Michael who kills Lucifer or the other way around, sooner or later one would make a mistake and BOOM! He dies. But then again, it's not like we are updated about their situation, all we know is that they are locked, so there's a lot of room for interpretations. Maybe they don't even fight there anymore, since the BIG fight was supposed to be on Earth.
Also I find it funny how angels fought many wars yet not many, including Cass, knew about the fact that angel swords can kill angels (remember in Season 4 how Uriel tells Cass "the only thing that can kill an angel, is another angel"). I mean they sure had angels who rebelled and fought against each other. But meh, maybe I'm looking too much into, it's hard not to find any flaws in a tv show/movie, it's not like they can be perfect. But just for the sake of discussion...