Earlier quoted context omitted.
I think here is where the problem isn't purely a technical one anymore. You're right that, from a legal perspective, there is a difference as the burden of proof would now be on someone else. But, if this actually matters or not, depends entirely on the situation. If you're a criminal trying to get away with something then the change in the burden of proof is all you need. If instead you're a politician trying to avo…
If you are a politician trying to get away with something, the public might have an interest in your secure messaging system being bad, but you yourself do not. Secure messaging systems generally take the side of their users, not the public.
Anyway, to further add to my point, depending on the context you don't even need to claim that someone stole your password. In the company where I am now , it is custom that, if someone finds out someone else didn't lock their computer, that someone sends an email (from the victim's account) to the whole office saying that the victim is going to bring cake to the office. DKIM is meant to prove that a message comes from an authorized server, but to prove the identity of the sender as well you need something more.
Edit: to be fair, I do get that with DKIM deniability gets harder. But I think that, for the average person, you would gain more in terms of spam and phising protection than what you loose. High profile targets have to take different security measures than the masses anyway.