>> whitespace on modern UIs ... Usability

Here here! But you're understating it.

Less information dense, and smaller fonts is an appalling use of type, and layout. These first principals about type, font, page layout, they are about readability, and scalability...

We have 600 years of printing, of typography to lean on. Thats 600 years of refinement on line length, spacing and size. The human eye didn't evolve in the last two decades all these lessons still apply!