> Sadly my eyes aren't as good as they used to be so I can't make a lot of use of the extra real-estate, but it always seems as if they're ahead when it comes to resolutions on consumer devices.

With proper 2x scaling as intended by Apple there is no extra screen real-estate. 5k at 2x scaling gives you the real-estate of a 2560 x 1440 display, just with a doubled pixel density and thus much sharper. This is the actual value of HiDPI display.