IMO your logic fits with large production runs.

In space applications costs can be exaggerated compared to actual production environments because the risks aren’t mitigated by something like the FAA, meaning they are often mitigated by some downstream process. Besides, a lot of the the designs already include critical spares, so there’s more than a single run, even in a one-off design

So build a copy out of the spares! If they're there, they're already paid for.

Not every part meets the criteria to be a "critical" spare. You don't have a complete set of spares for every component.

Are you suggesting that spares are only produced for the cheap parts?

No. Only for the parts deemed critical.

An example may be a part with a long lead time and no redundancy. If it is found defective during testing, they need a replacement right away and don’t want the schedule to slip while it gets fabricated

A lot of times when you make one of something, there's stuff you would make tooling for if you were going to do more than one, but which instead you do by hand. Sometimes this is even true in software, but not very often, because the advantages of being able to recompile something from source code are so huge.

Even when doing things by hand, making a duplicate alongside the original is not much more time. That happens to me every time I do something by hand.

This is true for things like motor mounts and pipe flanges, but not for things like optical surfaces and lathe ways. Even in cases where it is true, if you didn't make the duplicate alongside the original, you have to do the setup again to make a duplicate.

Yes, make it alongside the original. I've been saying that all along.

I think your mindset is stuck in thinking about this as an airplane production environment because that was your experience. And your points make sense in that type of environment. But research aerospace in which JWST operates is a very different scale, operating much more at the margins where the tech is less well understood and takes much longer to develop. Investing billions for redundancy on an unproven system is a big risk (besides the fact that it would likely be designed differently - and better - if started today).

The sunk cost fallacy already drives a lot of these projects...imagine how much worse it would be (and how many other, competing projects wouldn't get funded) if the costs were higher.

Doesn't help with telescope mirrors, as you likely know from experience.

I think the error in your assumption comes, in part, that there’s is tooling set u for mass production of each part.

A lot of parts are hand-fabricated. The setup may take time but it’s not necessarily like a assembly plant that requires new dies on each design.