I understand your point. However I think that in practice it's more likely to play out like the Greek Fire example in the article. That is, no we can't precisely replicate it, but it doesn't matter because we have napalm. Greek Fire specifically is unnecessary, since we can meet or exceed the spec with things we can make.

There are many different types of ultra-accurate magnetometer. They are made in many different ways. If one relies on some byproduct of some other industry that goes under and becomes nonviable, then the many competitors will fill the niche with a different type of magnetometer that will be just as good.

Even if we consider a different hypothetical industry that truly is single point of failure, well, there are academic papers explaining how these things work and how to make them. Maybe the specific model of metal is no longer made, but the instructions there and an intelligent, motivated person will figure it out. And then they'll figure out how to improve it.

I don't think the failure mechanism you describe will in practice lead to anything more than brief gaps in the availability of a technology. We can remake things to spec, like FOGBANK.

That kind of reconstruction effort only happens with a huge budget, which means for most of these technologies, it never happens at all. Academic papers only give a basic starting point; especially for materials science, people dedicate the span of their entire years to developing manufacturing processes beyond what ever gets published in public. With enough motivation you can have the pleasure of following their footsteps, but it will consume more than one lifetime to do so. And nobody will fund that rediscovery effort, because there's no money in niche industries.

A result of this is that, in some rare cases, there are certain machines and instruments where the best ones in existence were manufactured in the '80s and are jealousy hoarded in temperature-controlled vaults, because nobody today can make a better one and the industry that produced them is completely gone.

FOGBANK is a great example of a lost technology that had to be rediscovered at great expense, and that only happened because the military could write a blank cheque. But that's exactly a lost technology; they had to do the R&D all over again. And even then, they didn't have to make up for a bigger market that vanished; they were always the only customer for FOGBANK, so the economics hadn't changed. Imagine if the economically producing that material required a customer base the size of Kodak's peak market share to amortize the capital cost of a production run.

Most technologies don't have infinite money available to recreate from scratch; they depend on economies of scale and die out with their markets.