Ubuntu stopped being the lightweight Linux distro, and Linux Mint filled the gap it left behind
(www.xda-developers.com)
stopped
It never was.
Ubuntu used to be the obvious choice for reviving old systems that couldn't run Windows
It didn't.
Whether xfce and cinnamon require less resources compared to other DEs is questionable, but the question itself is dumb. Environmemt by itself has little value without programs that run inside of it, and most of those aren't cinnamon or xfce specific or adapted to them.