You have the advantage that all existing production Nix code is known to terminate in short order

You don't really know this - there are many code paths in nixpkgs that are never touched during Hydra evaluation (various overrides, user-facing flags, packages marked broken and so on). We think it might be possible to slowly move towards more static analysis of the existing Nix language, but this is a little bit further down the road.

(Note by the way that we have a "runtime type checker" for Nix, but it is mostly an experiment: https://code.tvl.fyi/about/nix/yants)