With only 10 large metro-areas destroyed, there are still other very large cities and metro areas. Phoenix, Houston, San Antonio, Dallas, Fort Worth, Jacksonville, all have over 1M people. San Jose, Austin, Charlotte, Columbus, Indianapolis, Seattle, Denver, Oklahoma City, Nashville, all have from 700K-1M. And there's 20 more with 500K+ people. We have a big-ass country. Military and national guard are spread all over, as well as "industry" that is tapped in time of war (we don't have much of it left, but enough to be useful)

I'm not defending whatever OP's point was, I'm just saying we would have a whole lot of people left. Very few resources, and very poorly distributed, but a lot of people. If we lost 40% of the population it would still be a lot of people.