Interestingly, he's not all that far off in a sense. Many commercial buildings used to be built to be multi-functional. The old school classic 3-story brick, with a shop on the bottom, maybe a workshop above, maybe residential (plus rentable space) on the third.

But once you choose high-density, you don't get much in the way of do-overs. For much the same reason that once you choose thinness as a positive metric for laptops, you lose a lot of flexibility.