Having played in that sandbox for a while I think that fear of the commercials from competition is/was real. A hatchery, whether upland or net-pen, can provide a consistent product of a consistent quality of a consistent size 24/7/365. That beats seasonal available wild fish. That said, any confined rearing whether it be fish or cows or chickens comes with environmental costs from concentrated waste, uneaten food, and the possibility of disease.

As to the question of whether net pens could relieve the pressure on wild stocks the answer is yes. But relieving that pressure puts commercial fihermen out of business.