Or overpopulated cities. Cities can be too large.
There is zero evidence any city in America is even close to this point. We're still on the beneficial end of densification: marginal increases in density improve outcomes on practically every metric.
Case in point: the densest parts of Manhattan don't smell. (Minus trash days.) Neither do Paris, Tokyo or Seoul.
Hint: It did smell. Every spring and a few other times each summer.
Zero evidence noise pollution is an unavoidable consequence of density. Plenty of New York apartments have better acoustic isolation than the average suburban dwelling. (If anything, as buildings get higher the problem gets simpler.)
I'd like you to meet my neighbors (both above and below) from the last time I was unfortunate enough to have to live in an apartment.