I haven't read all of the replies, but as a manufacturing engineer can tell you this (which many have already been brought up). That most of these parts are sourced and it would be the supplier that may decide of stop making the parts. In some cases it's because a company goes under, others they were bought out, maybe the car maker no longer uses them for the part(s). Once a part isn't in demand they stop making them.
I have come across this with $500K manufacturing machinery. The OEM didn't know their supplier had stopped producing certain parts on a 10 year old machine until we went to order these parts. The OEM had one of what we needed in stock and went to order more for their inventory and found they couldn't get the parts. Which they were still using in some of their product, so it was a shock to them also. They informed us of the issue, and we are still working out the solution.
Touch screens for machine controls. The supplier will make one huge run of screens, then move on to the next newer generation of screen. once that supply is used up, it can be very difficult to find a replacement or when you do, there are usually hardware and software issues that have to be solved.
We have bought machinery that even a few months later, a replacement part couldn't be found. In that case newer parts had to be made to fit the machine.
The problem is very few suppliers will agree to produce parts for length of times greater than a year or two.