The problem isn't just scaling between two different resolutions, it's the inconsistencies (yes, apps don't take advantage but that's not the only issue). For example, if I want 200% 4k (my monitor) and 100% 1080p (my 2 side monitors), I have to choose between ultra-tiny text on my 4k with regular text or blurry text on my 1080ps.
http://i.imgur.com/o1S8ZUt.png