I want to criticise either the idea that diminishing returns is important, or, at least, that dollar values make sense for talking about them.

Suppose we have a monster who likes to eat. Each serving of food is just as tasty as the previous, but he still gets diminishing returns on the dollar, because the marginal cost of the servings goes up.

We also have nematodes, who like to eat, but not as much. They never get a look in, because as the monster eats, they also suffer diminished utilons per dollar.

So the monster is serving the 'purpose' of the utility monster, but still has diminishing returns on the dollar. If we redefine diminishing returns to be on something else, I'm not sure it could be well justified or immune to this issue.

And, although humans are not an example of this sort of monster, the human race certainly is.