This approach creates a mismatch between singleplayer and multiplayer modes - this means that playing against a computer opponent rewards/requires different strategies than a human; a challenging computer opponent has more income and units but poor usage of them, while a similarly challenging human opponent has less income and units but uses them very differently; so playing against challenging computer opponents doesn't help you improve against other players but possibly is even counterproductive as you learn to adopt strategies that are bad in the other environment.

True. I don't know many videogames in which playing against the computer really helps you against human opponents.

I don't know whether a more capable non-cheating AI would help though. Not unless it specifically imitated how a (good) human opponent would play, which I guess is an additional and difficult to implement constraint.

I mostly agree with you - there is a little bit of a fine line though. I can learn from my opponents strategy playing against someone a little bit better than me. Maybe i lost because i need more or less of X? Tough to learn from when the opponent is creating the illusion of playing well, rather than playing well.