It's true that if all the people in Idaho, Montana, Wyoming, North Dakota, South Dakota, Nebraska, and the western half of Minnesota were packed into a tiny space with the density of NYC all of the land they abandoned would be better off, but the land they were shoved into would be much much worse off for it.
Like I said: the greater the number of people in a given area the worse it will be for that environment.
We need to find a better balance between stacking people on top of each other and destroying the ecosystem in the places where we live without any thought to biocapacity or local wildlife, and spreading out so far that it's impractical and inefficient to deliver or provide the goods and services humans want.
Since every human takes a massive toll on the local ecosystem, spreading that impact out more than we do in highly packed cities would be better for the environment and also better for humans who also suffer under those types of conditions. As I said, I don't think I've seen that balance hit quite right just yet, but it's something we're going to have to start thinking about more carefully as resources like drinkable water grow scarce, climate refugees from around the world are taken in, people along the costs are forced inland or to higher ground, and much of the western US succumbs to desertification and high temperatures that threaten to make large population centers unsustainable.
The US is likely going to get a lot more crowed in the future. We'll need to find a better way than absolutely trashing the land we settle on and suffering all the effects that go with that.