Not sure if that is "just a fact."
Do you have a source for that statistic?
Regardless, Pittsburgh's contribution was very large. If it had gone, other foundries and mills in the US would have expanded their output and capacity to fill the gap.
When the supplier who has the best cost advantage exits the market, the supplier who has the second best cost advantage wins.
There would have been a period of dislocation, but there would also have been strong incentive to shorten that dislocation period.
Replacing that in 1863 is not as easy as you seem to think.
The first coke-fired smelter in Pittsburgh was built in 1859, less than two years later it was the industry standard in Pittsburgh.
If the city converted its entire base to a new technology in less than two years, I believe that other areas would move quickly to ramp up.