[jnwatson]

While it is technically true, there's still plenty of vertical space. We've simply made it too hard to build new housing in US cities.

[jjoonathan]

That's technically true, but horizontal space can be parceled out somewhat independently, vertical space can't, and that's a big deal. You're right that it's a political stumbling block rather than a physical stumbling block, but city housing is expensive everywhere, so it seems appropriate to treat it as an empirical law rather than something that can just be hand-waived away.

I'd support efforts to build vertically in a heartbeat, I just don't have high hopes for them.