Having worked in a shop briefly, I would define it as "contributory negligence" on both sides. Shops don't want to have to source older parts/odd parts, diagnose problems because it takes more time than they can realistically charge for

Uninformed bike owners think "its an old bike and I don't want to spend and arm and a leg on it." but I want this old French stem/headset and Swiss thread bottom bracket replaced for $50.