The difference there is you're not violating some TOS or EULA by replacing parts on your classic car, and when you change your oil (do OS updates) there's no chance of suddenly your transmission refusing to allow you to shift gears until you perform more heroics and disable the artificial limitations.
Very few non-classic and/or popular cars receive massive aftermarket support for all parts - often the aftermarket supports parts that are in common with a lot of vehicles or are vehicle-agnostic (such as belts, etc), and in some cases you're plain SOL (try replacing an airbag on a 1993 Dodge Caravan, for example - all you can find are OEM used ones pulled from junkers).
I think your comparison would be more apt if, say, Ford disabled all vehicles that were 10 years + 1 day old. While Apple isn't disabling your OS, they leave you exposed without security patches, etc... - making it approximately the same.
