Given the knowledge available to them in the 70s, yes. A mistake, but done for good reasons (lowering satfat).
(And really, margarine can be plenty tasty. As a kid I actually preferred it to butter for some reason.)
https://youtu.be/reLIPoZQZ-8?si=lLXfhsdm89zlOWsI
Those commercials played multiple times a day in my childhood, and they never failed to piss me off, because they clearly demonstrated that "milk and a box of Chex" was not by any means a "balanced breakfast".