have there been improvements to the insulation? given how good high-end coolers are now, i'd assume that there's been something with the non-mechanical parts that could have improved
Ultimately, not much. The polyurethane insulation of 1995 is pretty comparable with the polyurethane insulation of 2025.
There are better insulations out there, but they cost money and are harder to work with. For example, we could theoretically vacuum seal a fridge, but that'd require an airtight seal and likely a stainless steel structure around the fridge.
I think a lot of coolers used to have some parts like lid or even sides that were simply double-walled with an air gap and no real insulation. I remember old internet posts about people enhancing theirs by drilling into these spaces and injecting them with a minimally expanding foam from the hardware store.
Maybe somewhat better insulation and then I have noticed with combined units that there is more of it. That is usable volume is smaller due to larger amount of insulation.
