We live in the 21st century, not the 18th century. If liberty requires that access to guns be a universal and inalienable right, then the capacity to manufacture and provide guns must scale with the population, and a small arms manufacturing infrastructure is necessary now to supply 300 million Americans with the access to firearms that the Constitution deems necessary for the US to remain a free state. Otherwise liberty diminishes because fewer guns wind up in fewer hands.

The Second Amendment does very much depend on the existence that industry, because if the government were to allow small arms manufacturing to fail, it would be de facto abridging the people's right to keep and bear arms. It's the only industry that has to exist in the US, by fiat.