> or have stocked ‘enough’ parts, where ‘enough’ is very hard to estimate up front.

On the building where I live, the company which does the elevator maintenance offered a significant discount on the replacement of the control system in the machine room with a more modern model (which uses less energy). Their reason: including us, they had only two clients left which used that older system, so once both upgraded to a newer (and more common) model, they would no longer need to stock replacement parts for that older model.