Indeed. Nixpkgs wouldn't have been as successful if it hadn't been using Git (or GitHub).

Sure, eventually you run into scaling issues, but that's a first world problem.

I actually find that nixpkgs being a monorepo makes it even better. The code is surprisingly easy to navigate and learn if you've worked in large codebases before. The scaling issues are good problems to have, and git has gotten significantly better at handling large repos than it was a decade ago, when Facebook opted for Mercurial because git couldn't scale to their needs. If anything, it's GitHub issues and PRs that are probably showing its cracks.