Some rurals are necessary for food and for providing estates to excess nobles, who can't all be housed in the capital. That is why I said 'to a large extent' and not a total extent.
Due to logistics, it should be easy to maintain a ring of rurals around the capital. Those together with the capital would compose a healthy core. I think that in order to break such a core, a very prolonged and concerted effort by two realms against a third would be necessary. Even then I am not sure; the logistical advantages of the defending realm increase as the battle fronts move closer to its capital, and it seems plausible that those defensive advantages could become so great as to preclude further progress by the attackers. I could see a whack-a-mole situation developing, where the attackers capture regions during their offensives, only to lose them during their refits, only to capture them again during their offenses, and so on. All because their refit times would be much longer than the defenders'.