I think annihilation is a property of how we played the game and not of the game itself. I get that the game was designed around it, but there's nothing stopping players in a Free For All from subduing an enemy into slavery instead of wiping them out, some may call this an ally. In fact, it's probably the optimal strategy, annihilating might be a pyrrhic victory, where one player is knocked out, and the 'victor' is left with half an army. The only winner in that situation are the rest of the players who are now better than the two players.
Except NvN games, I except FFA and 2v2v2v2 games to show elements of empires, it's only games with two sides where minmax becomes a viable strategy and damaging the enemy is the same as strengthening yourself.