But Apple charges ridiculous premiums for everything. For example, at https://frame.work/products/laptop-diy-12-gen-intel/configur... it's only $80 more for 32GB than for 16GB.

I don't think you should use Apple to put upgrade pricing in perspective. They've used inflated pricing to extract extra money from a captive audience for many years. Other OEMs cannot demand similar premiums for commodity products.

They use on-SoC RAM chips, essentially a cheaper HBM instead of just some random SO-DIMMS.

I think a better comparison would be Dell or Lenovo who do the same thing HP does, and they too charge a lot for what are essentially OEM versions of Samsung and Hynix memory anyone can get.

Well linux on desktop mostly just works. Linux on laptops has been a headache for a while with awful battery life, trackpad issues, wifi issues and things like that. So personally I don't feel the need for any dedicated linux desktop release compared to the laptop space where I appreciate more of a guarantee that the drivers actually work and I won't spend all my time messing with linux internals trying to get it to work.