I am sure that they can cover the 70% of American using their low frequency spectrum no problem. The only problem would be the relatively low customer count. They will have to partner with somebody to spread out the deployment costs.
Look in some places T-Mobile had a denser network and in others Sprint had a denser network and in others neither did. It's the nature of the beast. Let's hope that they do an masterful blending of the two networks and then buildout the places where neither was strong.