This is not a business concept exclusive to Marvel, or Disney. It's the same basic concept that all companies face, whether they sell toothpaste or cars or shoes or kitchen appliances or movies.
In the past few years, Disney has failed at that basic business concept with some of its flagship brands; Star Wars, Marvel, and now even Walt Disney Animation. Just like any other company whose customers don't live forever, Disney has to figure this out ASAP. I would suggest firing a dozen or so studio executives and resetting their entire corporate culture.
And stop taking business advice and consumer insight from the next brunch table in Silver Lake!