Any answer I give to that would be speculation. How about this: If MUAs evolve as a result of all of this, I would hope it's in a direction that doesn't just create whack-a-mole for the problem DMARC is trying to solve.
That suggests it would be in the DMARC cartel's interest to avoid putting evolutionary pressure on MUAs to do things they won't like. See you in ietf-822.
Regards, John Levine, johnl@xxxxxxxxx, Taughannock Networks, Trumansburg NY Please consider the environment before reading this e-mail.