I don't have proof or evidence, but I do have another way to phrase it that might make it easier to come across both:

There are only three "evergreen" ways to make money, when other people have access to your business model:

A. doing something too hard for anyone else to copy;

B. doing something too capital-intensive for anyone else to attempt (e.g. PayPal, SpaceX);

or C. doing something too distasteful for anyone else to want to touch.

All other markets will be commoditized.