Old spider-man content sells for a lot more than old calvin and hobbes content.

> Calvin and Hobbes was extremely mainstream. It only stopped being so because the creator stopped creating content for it.

I think that's exactly the point. It stopped being mainstream. Spiderman continues to be mainstream because the original author passed on the rights and now it will live for longer than the original author. Calvin and Hobbes will continue to shrivel in how well known it is.