If it could be programmed, having no direct city garrison at all and instead having that function somehow as an extension of your overall military force relative to population, while having territorial exchanges be primarily a matter of peace settlements diplomatically would be better, but that is at odds not only with the way that expansion works in-game, but also the whole cultural border pressure thing which is a major piece of the game. When I want something more realistic like that, though, I just play EU3. I kind of have to wear a different, more imaginative hat when playing Civ, which I'm fine with, but I see your point and agree if it could realistically be done within the parameters of the game engine.