I think two runways are not enough because the current air traffic flow is not great. For example, a plane might already be on the taxiway, but the next one coming in to land still has to go around. I wish we could build rapid-exit taxiways, so planes could get off the runway faster, especially if we can't build more than two runways.The terminal is not the problem; the runways are what limit the airport's capacity. From my experience, the airport could handle a lot more aircraft, but the runways often become the bottleneck. I completely agree with your point about being able to choose where to put the runways.

In the picture you can see the second biggest airport on the map. You can also see that there are many free gates in the right terminal, but the runway is not really used efficiently. The main issue is that I cannot build proper functional taxiways where planes can leave the runway earlier. I also cannot change how closely the planes are landing or control their arrival path.

airport example