> get good at things by doing them repeatedly and optimising

In an environment where information is shared freely and widely.

> So of course if we build a couple of naval ships every decade, maybe a giant train line every 30 years or similar, it will be done terribly.

That's a reasonable explanation but it's not exclusive. You can still face these challenges and not do a terrible job. There is plenty of evidence this is the case.

> Then there’s lots of hand wringing about how we can’t do this any more.

Of course we can.

> so far behind China in industrial capacity now.

We just don't want to pay first world wages for the work. The arbitrage has been beneficial for a few decades as long as you're not concerned about high quality capacity. Which is what you'd want for a war.