Nix isn’t just for reproduction. It has immutability so if you break your system configuration you can revert to a previous profile, and the way installations are managed allows you to install software that uses incompatible versions of the same dependencies at the same time.

In the 2020s we have new active USB to PS2 dongles like HIDman.

Windows 95 was also a shell on top of MSDOS. Windows NT wasn’t running on top of DOS, but it was primarily for business use until Windows XP.