Most rusted, or were discarded. They were forced to die.
There were loads of cars I, and my friends inherited, because the car was "old", a repair was $500, and the car was only worth $1k (this is the 80s, so 80s figures...), and the car had a tiny rust spot or two.
Yet that repair done at home, with a friend, could be done for 50 bucks and parts from a wrecker.
This is not survivor bias, these cars were in great shape, but instead for appearance sake, and "estimated value of the car" sake, people would throw it away.
I think there is likely a more throw it away culture overall now than in the 70s.
Cars started to last significantly longer when body rust-proofing improved (mostly in the 80s for American cars), when electronic fuel injection reduced fuel wash in the engines, and when anti-collision tech reduced the number of write offs of lower value used cars (ABS being perhaps the single biggest one, which prevents a lot of $2000 accidents from taking a $2500 used car off the road).
A carbureted, unprotected mild-steel car built in 1959, 1969, or 1979 was much less likely to be on the road 23 years later than a fuel-injected, galvanized steel 1999 model is to be on the road today.
We've mostly negated rust proofing improvements by using road salt that's more effective at getting everywhere and using salt in more places (you'll notice that the white crusted post-snowstorm hellscape was not a thing in the 90s or '00s). OEMs have gotten good at using plastic cosmetic trim to cover the initial rust points (wheel arches, rocker panels, etc, etc,) so that instead of needing attention after 10yr the problem can be ignored until a much later date when the rust finally makes it out from behind that plastic (and your car starts failing safety inspection if you live in an applicable state).
Agreed overall, though I think that last sentence is probably a very good thing.
If OEMs improve their product so that it does not require an expensive cosmetic rust repair at 10 years, but instead the cosmetic problem is hidden so the car lasts an extra 5-7 years before becoming a throwaway product, the consumer has a net win overall.
