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.

[0]: https://discourse.nixos.org/t/how-long-is-binary-cache-kept-...

[1]: https://hal.science/hal-04913007

[2]: https://luj.fr/blog/is-nixos-truly-reproducible.html

[3]: https://discourse.nixos.org/t/nixos-foundations-financial-su...

[4]: https://discourse.nixos.org/t/the-nixos-foundations-call-to-...