Fair enough, there are definitely some applications where old does not mean out of date (mostly non-internet-connected applications I think). The original example in the article contained "It can’t be upgraded safely because the entire world’s banking system would fall apart if it did.", which definitely does seem like a disaster waiting to happen.
Nix seems like it could fill a niche, but I do wonder if it will be as ubiquitous as some articles proclaim. Certainly with the apps I currently work on (Ruby/Rails mostly) we already experience almost none of the pain that Nix claims to solve, so I doubt many will switch.
Source code and abstract virtual machines make many things easier. However, I expect Rails is typically deployed via Docker or something to capture dependencies. That part could be replaced by Nix, in principle.
People also use Nix quite in combination with Docker, so clearly Docker doesn't go far enough on the developer experience, and Nix doesn't provide isolation so Docker provides advantages from that side.
I feel like there should be a more integrated, comprehensive solution for this sort of thing, but these are on the right track.
Nix seems like it could fill a niche, but I do wonder if it will be as ubiquitous as some articles proclaim. Certainly with the apps I currently work on (Ruby/Rails mostly) we already experience almost none of the pain that Nix claims to solve, so I doubt many will switch.