All private conversations shouldn't be published. That isn't necessary to hold people accountable for dangerous or violent speech.
Publication and repudiation aren't the same thing.
Yes, it might make it harder to punish death threats, but privacy is too important to sacrifice.
And not being a politician, and certainly not the one currently trying to hold the White House hostage, I don't see the public interest anyway.
If you think White House emails should be signed and archived indefinitely, that is one thing. That is not what we are talking about.
Mail being secure from surveillance is a foundational freedom.
I have no idea where you are getting the idea that we all should have to answer for what we send in private correspondence.
This discussion about DKIM is about non-repudiation and the ability to prove that a certain person sent the email.
If you send me a letter, I (or someone else who gains possession of that letter) should be able to prove that you sent the letter and hold you accountable for the contents. DKIM does that for emails.
Too, it's easy to imagine not knowing you need proof until some time after you receive an email.
If it isn't usable and enabled by default, it won't be used in practice - for the same reason almost nobody uses PGP.
In fact, it's quite common that the issue over unsigned documents in court is the interpretation, not authencity.
I think the issue of having to teach users how to opt in to signing emails in potentially controversial cases is preferable to having to teach them how to handle email communications that are permanently provable (for starters, never ever again leave out a quote and never ever write ambiguously).
If you got an email that warrants "holding someone accountable", you would have plenty of time before the keys are released. So if you receive an email and call the police, nothing would change.
What you couldn't do it save it for years and keep it as blackmail material / until it's politically opportune to use. Of course it's not as clear cut as that, and an email may look harmless at the time, and only later, with more context, you might realize it contains evidence of misdeeds. So even a good faith actor might unknowingly sit on evidence.