Not really. Destroying mills and foundries is a setback.
But taking Pittsburgh would not have meant an end to the Union's supplies of iron ore, or coking coal or water.
There were large foundries in Massachusetts, not as well-located as Pittsburgh, but still productive.
Pittsburgh's greatest advantage was Rodman's patented process which gave it a technological edge. That edge was transferable in an emergency.
I'm not sure how well you understand Pittsburgh's role in all this - it was not a magical place. It had great advantages which made it a very economical area to forge iron and therefore a place that attracted engineering talent.