This kind of argument ignores the real cost of having to upkeep a 50s car: the human suffering. Cubans are forced to use this option and their society suffers from it.
From an environmental perspective it’s a good thing. Don’t know if that makes it worth it but we are creating far too much pollution.
Cars from the 50s are less energy efficient and require more overall resources to maintain than a new car. They are also significantly less safe.
I imagine the mining of materials, waste from discarded car, etc. makes up for these things but I don’t know. I do think overall we are a very wasteful society and we cause way too much pollution.
A scrapped car is recycled for the metals in it. So if you recycle the old car you can count much less costs against the new car.
Keeping an old car functioning is only better for the environment up to a certain point. If your cars are staying on the road long enough, eventually it makes more environmental sense to replace a 50 year old car that gets 18 miles per gallon with a new one that gets 45 and can last another 50 years.