now that you mention it, yeah why didnt that happen? surely the impending destruction of canterlot is a little more chaotic than 2 fillies on a field trip getting into a fight.

Its explained he was released not becuase of that , but becuase Celestia's and Luna's Connection to the Elements was severed. So once Twilight and her friends used it turn him to stone he was permanently imprisoned there. Untill of course twilight and her friends can no longer use the elements

How i should have ended: Discord busts out, kicks the shit out of the changelings, "evil laugh", becomes good and moves to ponyville... just kidding, chaos rules forever

yeah i'd be thinking along the lines of discord showing up to a gigantic earthquake, canterlot falling off the mountain its attached to and being completely destroyed, luna and celestia being imprisoned in those caves under what now used to be canterlot and discord and the changeling queen, whatever her name is, ruling equestria and royally fucking shit up for all the ponies eternally, but thats just my view.