"Once you've scrapped a major facility ..."
Newport News and Electric Boat are hardly on the endangered species list. Newport News employs about 19,000 people and Electric Boat about 11,500. They both have excess capacity and are constantly lobbying Congress and the Navy for more contracts. The decline in Navy shipbuilding is the primary reason why we have fewer and smaller domestic shipyards, and that is the result of a *political* decision having nothing to do with free trade. Let the Chinese and the Nordic countries build low-value commercial ships (which have very few complementarities with military shipbuilding) and we'll focus on building the next generation of aircraft carriers and submarines.
"Harder to replace is skilled labor and intellectual capital once that goes away."
Here I agree with you completely. But I am certain that, for example, Newport News would not turn down a contract for another carrier on the grounds that it couldn't hire the requisite skilled labor. They have turned down (or, more correctly, declined to bid on) commercial contracts because they don't think they can make money on them *and* they know from past experience that they wouldn't have beneficial spillovers for their defense work.
The question is how we can compete in building boats. China is competitive because of the low cost of building. We are competitive because of our great skills in building all kind of vessels including the fastest military boat ever been in production. With probably on of the highest wages in the world.