This is an interesting analytical failure. To explicate. You go through your day wearing only boots. You see people wearing sneakers and so one day you buy a pair the wrong size and put it on with a tag inside and walk around like that for a day. You think to yourself how do all these people do this every day did nobody ever show them a good pair of boots! Sir Boots4Life's analysis is faulty they aren't all walking around uncomfortable because they don't wear the wrong size nor wear a small object between footwear and foot for extra penance and everyone isn't walking around with a fuzzy screen.
There as it turns out are other ways to scale a UI other than integer scaling factors. Even if you don't use svg you can use different image sizes and scale fonts by smaller increments yet.
You describe their being 1080p of usable real estate which is a complete failure to use a meaningful unit of measure. Your 5K display, future 8k displays etc use more pixels to draw a button 3cm x 1 cm. They aren't wasting increasing number of pixels they are drawing the element with increasing fidelity. If we measured screen real estate in pixels one would conclude that a 5" screen and a 32" screen both drawn in 1080p have equal screen real estate. This is a clearly incorrect conclusion. Clearly screen real estate is measured in area with fidelity not real estate measured in DPI.
Increasing fidelity might bear on the smallest element one can possibly usefully use but its not going to bear as much on what size element people desire to use which has much more to do with how far the object is from a users face.
You are saying that 4K monitors require one to choose between giant elements that somehow waste all the pixels or exceptionally tiny ones. This is silly. First off 108Op displays that range from 12" - 32" already had to adjust elements to be usable even before 4K became a thing and there were already more knobs than scaling factor to achieve the same end result. 4K changed the existing equation that has existed for 20 years by an exact factor of 2. 8k changes it by an exact factor of 4.
TLDR: Set an integer scaling factor then tweaking your fonts a little bigger or a little smaller until it looks nice according to your taste. Nobody on earth expected a 24" 4K monitor to display twice as much content as a 1080p 24" monitor because they want things to be the same size on the screen only prettier.