Do more people actually buy more soup because of Warhol?
I don’t know. But sometimes going after someone with a lawsuit will cause your own PR more damage than just ignoring the arguable copyright violation. IIRC that was the case with Kodak and Paul Simon. They figured that the bad PR from suing Simon would be more damaging than the copyright case. And Simon was saying good things about Kodachrome.
On the other hand waiting 40 years to sue is ridiculous. Copyrights are supposed to be defended as they happen. Else you can lose the copyright.