This has less space than 4k at 200% (normal for 15") or 1080p at 100%.
That's not my experience at all. For a while I was using a 27" 4k display at an effective resolution of 2560x1440. Obviously it wasn't as nice as 5k at 2x scaling would have been, but it was much sharper than a native 1440p display.
How does Windows fix this? I have used so many Win32 apps that are jus broken in one way or another on HiDPI displays, most shockingly Visual Studio, my experience of HiDPI on Windows is always worse.