I get my head around them by thinking they are bad? As in, not good. An undesirable property.
I mean I try to publish most of my interesting email conversations on the web, because every time you have a good email conversation that isn't public it's like taking a $100 bill and lighting it on fire. So I wouldn't ever personally use disappearing messages.
Literally the first rule of email is that if you wouldn't want it on the front page of the NYT then you shouldn't send it. The first national scandal involving email was Iran Contra in 1986. People should know by now not to put anything into an email that they wouldn't be comfortable with the entire world knowing. And while privacy is hugely important to individuals and essential for a healthy society, to me rotating DKIM keys feels like it's incentivizing people to use email incorrectly.
There is not a popular email system in existence that says
"To: [email protected] CC: Everyone [NON-EDITABLE]"
Quite the opposite is true. Gmail, for example, says "Google.com Mail protects your message during delivery As you add people to this message, this icon will let you know your message is secure."