I'd argue that back then it was obsolescence, probably even planned obsolescence. The Japanese automakers came in and disrupted the assumptions in Detroit, by building more reliable, more efficient, and longer lasting cars. Even if you wanted to keep a 1971 Buick for 10 years it would be difficult to do so economically, as rust started forming and the drivetrain started acting up.

And then in the 80's, facing pressure from foreign automakers, American automakers finally started taking quality control and reliability more seriously. Still, though, they've basically always been behind Toyota.

And you can see the difference in the used auto market. Back then, the older cars were sold for scrap. If they were still functional and economically feasible to run, they would've been sold to someone.

Some people can (and do) still buy new and replace every 3 years. The increase longevity actually ends up helping them with resale value on the back end of their ownership.