Currently nix-collect-garbage has an option to not delete older entries than given amount of time. It looks like that setting only applies to generations. If someone uses nix for development and rarely uses nix-env calling garbage collection will still wipe most of derivations. If there was an option to also check time when given store path was created and not delete anything recent it would reduce amount of packages that need to be fetched again and it would also reduce amount of data fetched from the caching server.