People moved. In the late 1800s, around 70,000 people lived here. Now, around 90,000 people live here.
In the 70s, the steel industry took a shit, and 30% of the working population was unemployed, then in the 80s the government closed down all the coal mines and it got even worse. Strangely, this area greatly profited from WW2. The shipyard and steelworks were so important to the war effort that it was the first place to be bombed by the Germans, even though it is pretty far north.
I myself moved out of this area and only came back a couple of years ago. It is weird here. It feels separate to the rest of the country, like time stopped while it carried on everywhere else. Unemployment is still very high and the last time I checked it is still the heroin capital of the UK.
People will always be spending and making money. Not necessarily in the same ways all the time, but humans are never satisfied.
Could you explain in more detail what happened to the coal industry in your area? Did people leave, or did everyone stick around?