I agree with Robin. We are do this hobby for preservation. Driving and demonstrating our restorations is the satisfaction. Sadly, sometimes a neglected or incomplete project has to be robbed so an almost ready job can be finalized. I support his quest, and eventually the donor vehicle will have given up its last (insert part number) for a good cause.
My brother was a wrench puller in the Air Force. He used the phrase robbing to describe the process of stealing working parts off a sidelined aircraft, called a hangar queen, to keep those on the flightline operating. It was a delicate balance hoping the supply system would catch up in time for the victim to be reassembled to fly to the contractor for overhaul or major work. If the crews took off too many parts, the work to get it flying would take much longer.
Unlike airplane parts provided by Her Majesty, for historic vehicles Robin is correct that the supply of replacements has been thinning out. Just look at Sherman tank rebuilders Balmoral Green (
https://balmoralgreen.com/les-blind-s/sherman-m4a1.html ). They are starting with rust encrusted hulks, and when the situation demands, installing modern bus engines. Running their tanks is more important than chasing down and acquiring the last (insert part number) in existence. Look at the lengths Wheatcroft and Littlefield go to put their treasures back together.