Your example is fucked.

First, Jacksonville is the largest city in the U.S. by area at 885 square miles, and occupies nearly all of Duval County, Florida. That's around 24.7e9 (billion) square feet, or roughly 3.5 square feet/person on earth.

Second, consider Tokyo--among the most insanely densely populated metropolises on earth. Population density is around 6800 per square mile, i.e. 4100 square feet per person.

That density isn't remotely achievable without large swaths of land that are much lower density, nor is it desirable.

Apparently your point is that there's plenty of room, because we could jam all current humans, elbow-to-elbow in Duval County Florida, standing upright, at a density 1200 times that of metropolitan Tokyo.

Epic fail.