Both great points. It also depends on the manufacturer’s continuing use of that part. There is an agreement to supply parts for 10 years for older vehicles. If the 2005 Vibe needs a new fender, it is likely long gone from the parts shelves. If you need a ball joint or control arm that is shared, or CAN be shared from a more recent car, that will likely be available. If it is a popular model, like a Ford, Ram or Chevy truck, the aftermarket will step up and supply parts for a very long time.
To @db4690 ‘s point… Electronics fail for many reasons. One that affects shelf parts as well as in-car parts are capacitors. And there are lots of capacitors in electronics. They don’t last forever and usage does not matter. That 20 year old brand new part may be just as much scrap as the one in your car. That is why the rebuilders exist and even they would ignore a unique model of car.
If you want to keep driving your 20 year old car, you might have to put up with parking your car for a month or more when it needs a part no longer easily available.