Capitals are the wealthiest regions on the island and there aren't enough nobles to fill all of the rural estates, so to a large extent peripheral regions are superfluous and can be exchanged freely without incurring much economic penalty. What matters are the capitals, and they are almost impervious.
My prediction is that there is going to be a long lasting stalemate situation, where economically superfluous regions are exchanged but the economic cores remain intact. This situation will persist until one realm massively screws up and loses its capital somehow.
Cities don't produce much money when they are starving.
