> HN still uses a font-size value that usually renders to 12px by default as well, making it look insanely small on most modern devices, etc.
On what devices (or browsers?) it renders "insanely small" for you? CSS pixels are not physical pixels, they're scaled to 1/96th of an inch on desktop computers, for smartphones etc. scaling takes into account the shorter typical distance between your eyes and the screen (to make the angular size roughly the same), so one CSS pixel can span multiple physical pixels on a high-PPI display. Font size specified in px should look the same on various devices. HN font size feels the same for me on my 32" 4k display (137 PPI), my 24" display with 94 PPI, and on my smartphone (416 PPI).
On my MacBook it's not "insanely small", but I zoom to 120% for a much better experience. I can read it just fine at the default.
On my standard 1080p screen I gotta set it to 200% zoom to be comfortable. Still LOTS of content on the screen and no space wasted.