Why does the author look at each city as a closed system? If we improve one city's community and rents go up due to increased demand, that necessarily means re

Why does the author look at each city as a closed system? If we improve one city's community and rents go up due to increased demand, that necessarily means rents went down somewhere else in the US. So, net-net, landowners didn't necessarily get richer. Furthermore, when the non-upgraded city eventually decides to invest in improving their neighborhood, people will move back, and the system as a whole will equilibrate.