No need to replace anything; the implicit assumption is a 2D space.

Then you can only talk about two concepts. I would like to be able to say, intelligence, morality, and obesity are mutually orthogonal.

If your model is 2D, then you are requiring a relationship between at least two of them.

The example was in 2D. The reader is able to generalize from there, we hope, no? :)

As long as you don't rely on concepts that don't generalize to higher dimensions. To the extent that you use concepts that don't generalize in your example, you are using a poor example.