It'd probably help to change it back to the original setup of early war planes being only available in the early war arena. Mid war planes only in the midwar arena and late war planes only in the late war arena. That way a person must choose the appropriate arena to fly a particular plane.
That way you wouldn't have people flying early war planes in other arenas. If they wanted their early war plane fix they'd have to go to the early war arena where they belong and that would pump up the numbers there. Same for mid war.