Dev: Sure, here you go.
User: But why is it so small on my new shiny tablet high density screen?
Dev: (SHit it worked okay for me) Okay now it detects the density and scale..
User: But when I move the window to my old good lcd screen it becomes way too big!
Dev: Okay let's see if I can dynamically adapt to a new monitor density, it's just one scale factor.
User: But when I put it on my big tv flat screen it is too small!!
Dev: (Oh shit you gotta be kidding me, the pixels are actually a viewing distance relative unit??!)
DPI scaling between monitors work exactly as you'd expect. Windows stay the same size when moving between high-DPI and regular monitors.
These two problems are two of the biggest reasons I don't use Ubuntu (or any Linux) desktop (I use a macbook with a headless Ubuntu Server box and, when necessary, X11 forwarding over ssh).
I have to log out when I plug/unplug or the windows will end up blurry or the wrong size.
I know they have a bad track record of not listening, but I think things might be different now, they seem to be a bit more receptive to feedback, particularly with the beta updates.
Or maybe I'm remembering the prerelease "hai where r the bugz halp" back when Win10 was not yet RTM...
(Admittedly, "points" are still likely a good measurement for print. Perhaps one can work backwards and fudge point as a measure of angle if you consider 12 point font at a typical viewing distance.)
I assume the real hard piece is figuring out the distance the display is going to be viewed at. Some definite defaults exist (phones are typically about the same distance away, same with desktop monitors) but unique situations certainly can exist. (I'm also assuming that the monitor can report it's physical size and resolution; combined w/ viewing distance, it should be possible to calculate FoV.) If you did this, you should be able to mostly seemlessly split a window between two displays, and have it be equal "size" in the FoV. (of course, some displays have borders, so that fudges it a bit.)
That is a good starting point for calculating the default "optimal UI scaling", but there are going to be adjustments needed for the FoV of the whole screen area (not per pixel) too.
With large screens, for example 24-30" on your desk, just the per-pixel FoV measure will probably be good enough. You have plenty of "space" for windows and content, and want to get the optimal scaling.
But once you get to very small screens like phones, there is a tradeoff between keeping font and UI sizes comfortable, and being able to actually fit enough content on the screen without endless scrolling. I am willing to strain my eyes with smaller font sizes on my phone than on my laptop, just so that I can see more than 5 sentences of text at the same time.
This is at the Operating System level, not like some random one-off application.
For me, the one feature I miss the most is a checkbox option for "Native Scrolling"; Did this really need to be removed?
And the vector-based competition to X (e.g. NeWS, Display Postscript) would have done better.