That doesn't make much sense because Nix doesn't prevent you from reinstalling, and after you reinstall, you are not forced to copy your old configuration.nix to your new /etc/nixos/.
In fact I think Nix is much better here. Instead of reinstalling you can just audit your configuration for anything that you don't want. You don't need to worry about stray packages, config files or services like on other distributions. And of course if you accidentally clean too much it is easy to revert via source control of your config file.
