I’m a vegan btw that uses Nix btw

Wayland for better multimonitor support, scaling, and tear-free rendering.

Yeah that thread i linked does talk about combinatorial explosion that would result in either a unmaintainable cache size or basically no caching at all of binaries. Your point about premature optimization is good. It makes me think that the seeming ignorance of USE flags by Nix was intentional.

Interesting, looked at the Gentoo docs to understand USE flags. Nix has similar capabilities, where some packages expose configuration options that apply to the build, but it’s not a overtly named feature consistently applies across all packages. It seems that something like USE flags could be implemented rather easily by Nix but was either deemed not necessary or was an oversight. You can still change the build for any package but it might involve introspecting the package definition to figure out what to change so not meant as a first class mechanism like USE flags.

Edit: found this: https://github.com/NixOS/nixpkgs/issues/12877

What is the value proposition of Gentoo over, say, something like NixOS?

NixOS. Declarative reproducible immutable systems are the future.