I just thought of something else with this: when going through reviews comparing different notebooks, it would be incredibly helpful if the RAM situation was clearly stated in a standardized place, ideally at the top in the specs for the reviewed computer (e.g. "Memory: 16384 MB, dual-channel, 32768 MB max, soldered" or "Memory: 8192 MB, single-channel, 24576 MB max, soldered + unsoldered"). And a list/spreadsheet of computers and what components are upgradeable and what the max RAM is, or adding that as a search filter, would be especially great, allowing users to just eliminate any that don't meet their needs quickly, without having to dig through a bunch of reviews. Not to mention it would help discourage the companies from doing this, as doing so would mean their computers would simply be ignored by people.
As for what else would be nice to be able to upgrade or swap it out, there are a few things I think would be interesting, though obviously they would add to the cost and complexity. A swappable keyboard would be nice, as would a port module that could be swapped out to provide different combinations of ports (more USB-A or USB-C, HDMI/ethernet/etc or not), allowing the user to choose what they want and change it later, e.g. getting 2 USB-A and only one USB-C but later switching to 1 and 2. Even a replaceable webcam module to upgrade it with a newer, better one after a couple years. And of course external batteries. That's a feature I sorely miss from my old Dell laptop, where I could just have a spare battery or two and swap out as needed. Of course, that wouldn't fit in to the modern desire for ultimate thinness, and it would be trickier to do with a convertible, but it was nice. I always thought with laptops and phones where you could swap the battery they should have a small internal one to allow for swapping without powering down.