The 512Ke is still supportable. It will run 6.0.x without a hitch.

While it is unfortunate that Apple does not offer System 3.2 or a similar version for the 128K and 512K (and Mac XL) I am willing to bet that there are enough original system disks to go around. There's only 900 or so of us on this forum and I'd imagine there aren't too many more beyond us who use and collect vintage Macs. If there are still 3500 original disks left it should meet demand. The trouble is finding them!