You are the one who is not understanding. What you say is correct about window chrome and OS widgets, but those are not what uses the bulk of the screen real estate for most activities, such as writing code in any editor using a terminal program, or creating diagrams. The user is in charge of the font size or zoom ratio in those cases, and with decent eyesight you can see a LOT more text in the same area on this new high-density screen.

If, on the other hand, your work involves mainly pressing OK or Cancel in modal system dialogs, then you are right.