"America needs more dense cities like NYC, for economic, national-strategic, and environmental reasons. "

Not sure what you mean by national-strategic, but sure cramming people in like sardines makes it easier to make things efficient - but it's very realistic to make less dense populations efficient/sustainable too. Urban developement isn't some kind of ecological optimization problem. There are factors you are completely dismissing, like overall happiness, contribution to the community and the nation as a whole, cultural value generated etc.

The culture and appeal of SF are contingent on it staying small.