yeah, but even in 1918 not every city had a refinery or blast furnace. And even back then, a lot of the basic stuff was imported (think opium, cocain (probably the top medicines back then...), rare metals...). On top of that, in 1918, a lot of the things you (and I) take for granted were not even found (for example antibiotica). And instead of random factories, each city (heck each small town) has like 10-15 CNC-machines ready to produce anything which was done in 1918 - a problem might be all things chemistry/mining related, but the real important stuff (which was known in 1918) also back then has been produced centrally (and still is)...
The hard part is the basics - things that get commoditised tend to get manufactured more efficiently, and at massive scale this tends towards centralisation.
As a concrete example, in the entire country of New Zealand, no one manufactures window glass. Every window, everywhere in every building, ultimately gets shipped into that country in a container.
We'd also miss shoes as there are no "real" factories locally anymore. I think we make nails but I can't tell if we can really make bolts. So I'm not talking about cars, computers or aircraft. No way. Windows. Shoes. Bolts.
So OK, we're missing commodities, most industrial chemical processes, feedstocks, experienced manufacturing labour and plant expertise, all of which went south when NZ was one of the first countries to drop its pants and remove import tariffs. OK. I don't have a dog in that fight, there are reasonable arguments to stop subsidising things you'll never be internationally competitive at.
That said if all imports stopped tomorrow for, let's say, 2 years, it's surprising what you can do without or improvise. The main thing I think we'd really miss is life sustaining medicine. A loss of exports would actually be more catastrophic since our farmers would a) have no reason to exist and b) not be able to keep the finance wheels turning.
We're unbelievably wealthy compared to people in 1918 and we have a lot more slack and fat in our systems than we really know.
> The main thing I think we'd really miss is life sustaining medicine.
Yes, we need flexible chemistry machines on the style of CNC mills. The good news is that they aren't that far away, at the next pandemics we will probably have them.
you didn't have those meds at all in 1918 either (and this was OPs point). and for the NZ case: did NZ ever have a real industry? With a 4.5 Mio. population today I somehow doubt that... As for basic industries (and meds): in western europe at least, we still have those things, small-scale and specialized, but we have it, including all the supporting industries (what we don't have is electronics, which 1918 wasn't a thing yet ;)). We needed to import the raw materials for 200 years. So?
I have nostalgia for local production but also recognise it can be harmful to subsidise inefficient industries. Hard to draw the right line between cronyism (obviously tariffs are actually a tax on locals) and resilience. Strategically, NZ unilaterally dropped a lot of tariffs on the advice of economists without considering, game theoretically, how to extract corresponding concessions with trading partners. This started in NZ in the late 80's. They then negotiated largely empty-handed (What will you give me? Wait you already did that.. nothing else?) and had a lot of trouble getting other people to drop their tariffs. Which are still there, in many cases! So there's my dog I guess?
