It's quite common for people to reach child-raising or retirement age and move to such places, or even leave their families there and continue to work in the big city. These cities also tend to have a lot of cottage-industry scale businesses: the average small American town is probably the worldwide capital of some incredibly obscure product or service. A town needs hard-currency inflow to pay for their imports but not a terribly large amount of it.
Consider where the money to pay the plumber comes from. Will hundreds of people from other towns call on that plumber? Or on their own plumbers?
What I'm trying to say is that at some point, someone, not necessarily the plumber, but someone has to bring money into the town.