So, where do I start?
1. This thread is entitled "singleplayer experience". But you say multiplayer is the ONLY way to play CC. And you own an entire website (CCS) where you can tell people that. Instead you say it in this topic, maybe that makes you a multiplayer chauvanist? You want to do multiplayer, there is nothing stopping you.
2. CC has great graphics, and a ton of data that can be interesting to lots of people. And people can play all sorts of ways. Single Player games have one unique advantage over Multiplayer games. You alone decide, the time limit, the speed of play, additional victory conditions, battlegroup compositions, and whether or not YOU ALONE have met those victory conditions. All these things generally cannot be done in a Multiplayer game.
3. You say the AI performs poorly. I say the AI performs way beyond my expectations. And the AI has limited memory, and applies a single intent to its approach to most games. And for some people, just observing the combat can be very satisfactory.
4. You say the AI performs better on smaller maps - "Simply because the fight was concentrated." SIMPLY, there is your bias showing. You mean on a smaller map the AI teams can support other AI teams in width and depth. That is not very simple, it is the root of combat in the western world since antiquity and I doubt there is a object of methods in CC for the AI to plan support for its teams.
5. The campaign game, over time, has had the battle maps expanded to enhance the campaign game, but in so doing, it comes at the expense of the single player experience. It was an issue, the game designers didn't even know was happening.