Yes for me the challenge is beating the AI.
Once you get the upper hand in most RTS games that's it - it's just grind from then on and I usually don't bother to finish it off.
Civ is a bit like that - once you have got to medieval/industrial age there are few strategic surprises left.
What interests me about AI war is that it looks like I can get off to a great start but still not be sure of winning the game.
My current (first proper) game on AI 5-6, I have had a trivially easy start.
Plenty of resources, 3 adv. labs within 3 hops and only one planet scouted that I can't immediately steamroll which is stuck at a dead-end of the system anyway and which I've just poured turrets around the entrance to.
But my AI progress is up to 80 with only 3 planets taken and I know it's going to get harder not easier as it goes on..