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.
¿Some? ATM machines have hard disks, and are certified with specific models.
You would be surprised what a hard disk with 1/100 the capacity (or even less) of a $100 one costs.
Said business would eventually declare bankruptcy and stop making the parts
One difference is the difficulties in making some 'parts' now. It's one thing to make a fuel pump for a 1950's car, quite another to duplicate a controller board, often with obsolete parts and secret software, that runs the convertible top.
Another is that a lot of parts are more monolithic than they used to be and harder to design/build. Look at headlight assemblies.
It doesn't help that with the increasing amount of software and electronics in a modern car (a Tesla being the leading example), the auto industry is acquiring the product lifecycle times and forced obsolescence of the PC industry.
Cars would get extremely expensive. Imagine Ford still having to supply model T wheels and engines.
They either would have to have kept a production line and employees knowing to operate it around, or have stocked ‘enough’ parts, where ‘enough’ is very hard to estimate up front. Also, stocked parts deteriorate, so for, for example, tyres, they would have to keep a production line running.
And that’s the easy case. At least Ford can ballpark know how many model T’s still exist. what if a car of a model that we thought didn’t exist anymore gets discovered in a barn?
And of course, that wouldn’t work in cases where manufacturers get bankrupt.