By making the DKIM keys public, you are converting solid evidence of something that was said into something that was either really said, or someone else pretended that they said.
Evidence was destroyed.
Evidence was destroyed.
Which is very distinctly different from a passive act of not maintaining evidence of the origin of every single thing. Keep in mind that no data is altered - the equivalent of all collected samples remaining intact.
It's still just as possible to collect email logs, their contents do not magically dissappear. They would have to be actively manipulated by the party which holds the copy that would be provided to the police (either reported to them or confiscated, etc). That same party could already decide to delete the emails or strip signatures and then alter them.
Edit: Removed the word "literally" because it was incorrect and caused distraction from the actual argument.