Iron Man being a super-genius is essential to his appeal. I have no interest in reading about an average joe wearing armor he can't even understand. Rhodes or Castle, it doesn't matter to me. They don't have Stark's super-power, so I don't care.
That's one of the problems with how Marvel treated Rhodey. If you look at his background, he's also a pretty smart guy. He's supposed to be an aerospace engineer. Yet, they either wrote him as just a brute or a servant.
There was a huge opportunity for Marvel that they missed.