This fails the role-reversal test. If Donald Trump had his e-mails leaked in 2016, the blackmailer would likely have extorted a ransom payment from then-candidate Trump. He paid off Stormy Daniels, after all. Nobody would have gotten any juicy e-mail dumps, and some criminals would have had actual leverage over politicians. Just because Hillary Clinton was less shrewd than Donald Trump does not mean that blackmail material is good for us as citizens just because some politicians don't pay ransoms on principle.
If you want transparency from your politicians, then you should demand unconditional archival and publication of campaign e-mails. Build transparency into the system. Leakers are not archivists, nor are they journalists. They are leakers, with an entirely different set of motivations and incentives which only sometimes align with journalistic or archival motivations. You as a member of the public will not hear about leaks if the person in possession of those leaked files has successfully extorted or ransomed the politician they came from. In this particular threat model, DKIM does not provide a social benefit to you as a citizen, it provides a monetary benefit to the leaker.
If you want transparency from your politicians, then you should demand unconditional archival and publication of campaign e-mails. Build transparency into the system. Leakers are not archivists, nor are they journalists. They are leakers, with an entirely different set of motivations and incentives which only sometimes align with journalistic or archival motivations. You as a member of the public will not hear about leaks if the person in possession of those leaked files has successfully extorted or ransomed the politician they came from. In this particular threat model, DKIM does not provide a social benefit to you as a citizen, it provides a monetary benefit to the leaker.