That should do it. Run as root manually or include in your package as the prerm/postrm script. I think that's better/cleaner than leaving a backup file lying around, which may be out of date by the time you remove the package. This should just remove the two exact-match lines the original file had added with my example script, and add the original gainroot-allowing line back in, at the same spot where it originally was (unless new lines were added since then to the start of the file).

As for why I didn't originally include this: In general, I completely agree with you. For this particular example, however, my goal was to get myself root - I don't intend on ever taking this root-ness away from myself, and the .deb package is, in this case, just a means to an end - in my mind it's a one-off-manual-tweak-in-the-form-of-a-package, not a true package. And tbh in my mind this is like applying a bugfix to a system package.