[jakobson14]

You have to factor in "oh shit, everybody who knew how to do that kind of weld retired" or "oh shit, they don't make that kind of valve anymore" as well. The older shit is, the harder it is to find expertise and spare parts.

The sea dragon concept from the 60's is completely impossible today because they literally don't make the kind of cheap steel plate it was designed to use anymore. We can't build saturn V engines anymore (despite extensive blueprints) because those who knew how to do the extremely long, perfect welds involved are all dead and that kind of weld has fallen out of fashion so the skill no longer exists.

You frequently end up re-engineering each bit of "old technology" for the current era's materials and skills, unless you literally maintain an unbroken chain of continual manufacturing all the way along. We stopped building new space shuttles 40+ years ago.