This is undoubedtly true in areas like New York, but in most of the United States, the trend hasn't be to build up, but build out. Spurred on by the belief that cheap gas will always be there.

And as for the necessity of the suburbs.  Americans have this inane plantation ideal that makes them believe that they need a patch of grass larger than an old peasant's farm plot in order to survive.

What's wrong with public greens and parks?  What's wrong with having a small back garden with just enough space instead of a small field that required constant maintenance even though its a monoculture?

I think that simple zoning laws, like relaxing setbacks, and mandating a 1-1 match for new commercial space with residential space (matching the neighborhood income spread) above the store.  Would do a lot to change things quickly.

I don't think that urban living has to involve gentrification.  What's needed is concerted government involvement to ensure that the demands of the community, not the market alone are met.

And I'll give my consent to any government that does not deny a man a living wage-Billy Bragg