This seems like a surprisingly bad idea. I would think the vendors' engineers would be able to identify the parts that qualify to be 3d printed, given how well the expected material characteristics match what the printer could produce.
I hope their plan isn't to 3D print safety critical parts like the rotor blades, that would certainly end poorly in the way you mention.
“But digital models simply don’t exist for vast inventories of older parts, some dating back decades.”
I presume that this has something to do with their approach.
These arn't irregular shapes. You can draw them from the part if you need to