So hardware that may still be perfectly usable but predates NVMe should be tossed out then?

The exact same thing you already have to do to upgrade the memory on such a computer, you go buy used/old stock DDR3 or cannibalize from another system. Pre-NVMe systems are DDR3 era and older. Time goes on, interfaces update and anyone looking for compatibility with their older system will need to either use an adapter or buy used/old stock. If there’s enough demand like with motherboards there can be a random Chinese brands making new hardware for old platforms using a mix of new and cannibalized parts