From experience just maintaining a private 1000+ package overlay to nixpkgs, this sounds super fraught, at least for any of the scenarios most typically interesting to developers, like wanting a backport of some library version that just landed to your otherwise-stable months old pin of nixpkgs.

For a lot of the most important ones, though, nixpkgs already maintains multiple recent versions, though— fifteen boosts in there right now: https://search.nixos.org/packages?channel=unstable&from=0&si...