But among the "new" and "widely used" apps on my desktop right now that don't scale well (i.e. bitmap scale) are e.g. Skype, McAffee, Cisco AnyConnect (it seems about 50% of the apps on my desktop are not properly DPI aware, but simply bitmap scaling).

That's not the bad part though, I can learn to live with a few blurry bitmap scaling apps (and the odd miniature one that doesn't scale at all for some reason), but the bigger problem is apps that partially scale so they become unusable. Here is a screenshot I just took, of JetBrains DotTrace (a profiler) at 2x scaling (At a laptop with 15" @ 4K): http://prntscr.com/i4tpmb It not only looks terrible, but some things are actually unusable.

The bad thing about that kind of bug is that when an app is unusable, even if it's only one app of 100 that you use, you have no choice but to change scaling or resolution just to do that task which is a terrible interruption, especially for scaling that might require a logout to take effect.

Edit: I don't consider multi monitor with different DPI's a fringe feature, since the high DPI is very common on laptops so any setup with a laptop plus an external screen will often be using different scaling for the laptop screen and the external screen.