Huh? Based on what? The packages I use keep getting updated, so clearly nixpkgs is alive. The OS I use keep getting updated, so clearly NixOS is alive to. Who exactly is dying, and what makes you believe that?
Nix tends to have updates almost as quickly as Arch does, I run a lot of bleeding edge software and have no issues, and the breadth of packages is immense. Nix just has a different style. With the AUR, you can adopt an abandoned package more easily. That has pros and cons (the AUR has recently suffered multiple supply-chain attacks due to packages being adopted and infected).
I'm fairly certain that the graph you're thinking of is complete bullshit. It only had approximately a third of the package count listed for a particular Ubuntu version than said version had in actuality.
Source?