It's difficult to pull this off with every website and keep it readable, without any hint from the site. This is why we have CSS for different screen sizes, rather than relying on mobile browsers to make all the decisions, along with an option to view the site in "desktop mode". I do agree that it would be annoying if sites made this decision for users.
But I think the more promising use for this could be enabling aesthetically pleasing color palettes that are power consumption optimized. Picture a HN that switches to darker tones of the current colors when you're on a mobile device.
