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.
No, destruction of evidence involves things like making something impossible to analyze and evaluate. Publication of a key doesn't erase the original messages and does not make it impossible to look into their contents to try to establish authencity by external means. Causing ambiguity is not destruction of evidence.
That would be an act of submitting false evidence, where you actively make a false claim regarding who the sample belongs to.
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.