Funny thing about that: if you want them to be scaled correctly based on different factors for each monitor, then you need a compositor...
You and I have an opposite definition of "correct" in this context, perhaps neither of us should use it! See enriquto's comment above, this is subjective. To us, we expect a window of size WxH to be WxH pixels, regardless of display. That's my definition of handling it properly, and when I use Wayland that's what will happen.
I will (grudgingly!) concede that perhaps it's not "universally and obviously correct" that things stay the same number of pixels, if you concede that perhaps it's not "universally and obviously correct" for windows to be drawn in terms of points and scaled behind the scenes.
Before this discussion I didn't have the concept of "scaling" my windows.. because that doesn't make sense to me, and it's something I'd never want. So my scaling factor for everything would be 1. Given that, Xorg "scales" perfectly.
No, you don't. And using a naive compositor is liable to make things blurry compared to the direct approach (the application requests the physical dpi for its current location and changes its vector drawing).