Your entire argument falls down by the games you listed. The only game where the AI is adequate is Halo CE, because, still to this day, it hasn't been bettered. Even by its own sequels. The addition of enemies who by design were just bullet sponges hampered the latter games. (as an aside, the moment in Halo 3 that summed up the game to me was early on when you first encounter squad of Brutes in a corridor in the base. I engaged them, then fell back, expecting them to give chase and fuck me over. But they just milled around in their initial area, allowing me to snipe them all from afar. Really disappointing AI).
Combat is the worst aspect of the HL games, improving with each sequel, but still lacking the highs of Halo.
At the other end of the scale, COD games are just light gun games dressed up in FPS clothing. The AI is super cheap and entirely predicable.
If all games had AI as exciting to fight against as Halo CE, you'd be onto something. But they don't, so you're not. I also remember reading an interview with one of the Bungie guys who said the introduction of moveable scenery in Halo 2 confused the hell out of the AI, which prevented the classic emergent gameplay firefights from CE with Elites.
