Classic cars can still get parts after 25 years, but a lot of times its custom machine shop stuff. There is a market for it. Really there's also a market for restoring old computers (look at The 8-Bit Guy, LGR, The Living Computer Museum (Seattle) and all those other people who restore old hardware for fun and education.
But I get your point, those are special cases for preservation. For mainline things, especially with today's processes for continual integration, dependency checking and advance build tooling, dependency rot is something that should be accounted for in all software project plannings. If your dependencies are a few months out of date and you don't have the time to update them and re-run tests (people write tests right?) things are just going to hurt more and more later.
But I get your point, those are special cases for preservation. For mainline things, especially with today's processes for continual integration, dependency checking and advance build tooling, dependency rot is something that should be accounted for in all software project plannings. If your dependencies are a few months out of date and you don't have the time to update them and re-run tests (people write tests right?) things are just going to hurt more and more later.