Question

A large city population was thus both a threat and an opportunity

Answer 1

Large population in a city, can be a bane as well as a boon.  

Increased pollution, cramped living quarters, limited medical facilities for the large population - these are some of the problems faced by cities with large population.

At the same time, a large population brings benefits for the city, by way of increased business, increase in civic as well as commercial facilities etc.