They emptied out because businesses transitioned manufacturing outside of the major city centers.

The 'transitioned because the price of being in a city was too high. The businesses first moved to the American south where the Unions were weaker, and then overseas. I live in New York and you still hear stories about how fast it happened. "SoHo" was a name dreamed up by the artists who started moving into the empty factories. All the folks who'd been priced out of Greenwich Village started moving in.

Also, Vietnam didn't 'grow' the economy. It was more like steroids. The old steel mills that should have been renovated years before were now running 24/7 to make enough bombs.

When the US mills couldn't produce enough for the Germans and Japanese markets, those countries started building their own plants. These new plants used way less oil than the older American plants, so when the Oil Crisis hit German and Japanese cars became a much better bargain.

If Nixon hadn't been pumping money into the steel industry it might have transitioned to lower cost plants on its own.