It solves a specific instance of the problem of needing packages from two distros, which is a common instance of that problem: wanting a stable base set of pack

It solves a specific instance of the problem of needing packages from two distros, which is a common instance of that problem: wanting a stable base set of packages on your system with the ability to use some newer, up to date ones. Nix is highly relevant because it solves that case: you can run Nix on any stable distribution like Debian, and the Nix package repository contains a very large amount of mostly up-to-date software, similar to the Arch+AUR repositories. Another problem it solves more generally is getting access to a large repository of maintained packages from a base system that only has a small repository, even if those packages are up-to-date. Both of these are reasons you might use Bedrock, so it doesn't seem off-topic.