Truck parts have been made mostly in high volume manufacturing facilities to produce parts at the lowest cost. That worked simply because the volumes produced were large and the huge investment in plant and machinery could be justified by giant volumes.

As competition increased and volumes of the big car manufacturers dropped, the result has been bankruptcy and plant closures. There are many factors that contributed to the failure of the manufacturers, but the resulting plant closures have many effects.

Spare parts for the big US car companies have mostly come right out of the high volume parts plants. These plants were either inside the Big 3 companies or outside. In either case, the spare parts for older trucks got a free ride, so to speak. The plants were there cranking out large volumes, and it was easy and cheap to maintain tooling and documentation to make a few spare parts in batches. These parts might even be stored in small volumes and sold a few at a time.

With plant closings, this cheap source of OEM parts has in many cases suddenly disappeared. Not that the parts cannot be made somewhere else. They can. The problem is that the documentation as to how to make the parts and even what the part is can be easily lost. Then what?

Loss of engineering documentation doesn't mean the information has disappeared, it means nobody knows where it is. Remember, many of these spare parts are not documented on computer data bases. They exist on paper drawings and those drawings may be almost anywhere. In many cases, it may not be the product documentation that's the problem. It may be the manufacturing documentation that disappears.

If the product information exists, the manufacturing processes can be reinvented, but only at high costs in many cases.

Cheap truck parts existed because of high volume manufacturing. As that disappears, there must be a different approach to supplying parts for older vehicles. My guess is that many parts will be unavailable in the near future and as a result older vehicles will die before their time, simply for lack of maintenance parts...