How far gone is too far gone?

We cannot fight the cancer (rusting steel) and save them all. It's sort of like being a organ donor. You would be surprised what some Classic car insurance companies will pay for the smaller pieces that aftermarket does not have.
I had a 67 B'cuda vert project laying here for a lot of years. This was a originally a slant 6 build and I had spent a lot of coin on parts. Another owner needed very nice trim parts I had here to finish his car, whatever it was per the tag. It may have been Formula S. I don't know or care. I let him have it at a decent dollar figure so he could finish his car.
Spend a small fortune on sheet metal and bodywork to save another, or support the hobby.
We see so many classics at car shows, etc.., that realistically should have badges on them today that say Made In Mexico, Made In China, and Made Some Damn Where.
There are more than me who prefer OEM made in America parts. That is all my buyer got from me. Exactly what he was looking for.