If you want to transfer assurance of the authenticity of an email to someone else, you can do so without DKIM; just sign a timestamp or something. The problem with current DKIM configurations is that it provides that assurance to everybody, including strangers who have no business having it. Which is why the ask here is for Google to do with DKIM what OTR does with MAC keys: burn them periodically, so that only people who have explicitly arranged to share authentication do so.