I certainly get the economics of it all, larger airport with a combined catchement area, offering more/ regular flights and more destinations.
At the same time, it'll never happen, for the exact reason NH117 has mentioned, no one is brave enough to bite the bullet. You need a huge plot of land with enough room for at least 3/4 terminals, 2/3 runways, parking for 5/10k cars and road/rail links. Imagine that getting passed through the governments budget would be almost impossible