Importing something is better than nothing, and this achieves the goal of keeping popular configuration envars like DXVK_FRAME_RATE. IMO, it is absolutely worth the code bloat.
Code bloat is not an issue, I'm more concerned about the unpredictability of which variables will be removed.
Maybe now that we have b1e94119f07919991edb1e629090b04e7e8b34a9 we could be more aggressive in removing unneeded vars. For instance it's very unlikely that NIXPKGS variables would be needed by a Windows app.