Manhattan has fewer residents today than it did in 1920. It also has much less building happening, and rents are higher now than they were in 1920 for the same square footage. Some of that is we don’t allow people to live in new boarding houses, some of it is that we downzoned the whole city multiple times, and there are a myriad of other reasons we should tackle as well.

It may not be physically possible for the entire world to live in NYC, but we surely could double in population without breaking any real physical constraints. Besides the political constraints that make it absurdly expensive (in time, capital and social power) to build for more growth.