Oh, indeed, Boromir has many good qualities, but I'm not sure he really has what it takes to be the head of a large nation engaged in a long war with a superior power.

Fair comment. I can see a situation in which Boromir would make a popular and well-loved figurehead, inspiring people in time of war, and Faramir would fill in many of the other gaps. Sort of like Eärnur and Mardil - hey, and we know how well that turned out!