CD-R and CD-RW discs use different methods of data encoding, and were never advertised as having similar longevity.

I mean...sorry, but this is wrong. Mitsui CDRs for example are still being sold on retail sites advertising 100+ (and sometimes 300+) year longevity. Similar to bitrot, internet rot makes it hard to find advertisements from the 90s, but I am comfortable that was true then too and not limited to "gold" high quality discs like Mitsui.