Look at the legal industry for a long time people couldn't hire lawyers because there weren't enough and now there are too many lawyers and their salaries have gone down. If there are too many doctors in a city they will get less business and make less. The difference here is it is a monopoly controlled by the government so their is no competition but their is immense competition from lots of qualified people every time a firefighter position opens up that means they could easily lower the wages and still get enough qualified people to fill them.