I believe in most cases the structure value does go down (or at most stay constant), it is the value of the land that is going up.
In principle the supply side has natural constraints, but they are easily 2 times above what has been built until now.
You can think of it this way: the population density of Manhattan is about 27000/km2. The entire NYC has a density of about 10000/km2, while Jersey city has 7000/km2.