You would be buying cars for a heck of a lot more money that is what and the manufacturers would have to put every last living cent into making sure their cars are DEAD reliable for a "reasonable" amount of mile. Reasonable is, as many miles as it takes to justify getting rid of a car that has never broken down and may or may not be able to be sold as there will be none too many buyers who want a used car that they do not know the history on that has no parts support. That is, unless they wanted it for parts because of the aforementioned.
Car companies make most of their money off service parts. Cars have been around awhile and they have a long history of needing parts here and there and still being perfectly acceptable vehicles.
Guns have a long history of being very reliable and needing few if any replacement parts EVER for the average owner. There are exceptions though as the OP makes very clear.