Nix cache (cache.nixos.org) does in fact maintain older versions[0]. In fact, they maintain so much older stuff (binaries and associated source), that they are used in both research[1][2] and are having issues with gigantic cache size[3][4].
And yes, their reasoning implies NIH and just unfamiliarity combined with unwillingness to really understand Nix.
Even if this weren’t true, setting up their own binary cache on S3 would have been trivial. It took me half a day to get it set up for our CI pipeline.