> In which scenario Amazon would deny sending an email and you would be protected by DKIM?
You want a specific scenario of a dispute between a vendor and a customer? Ok. Let's say I email Amazon's customer support to ask them if a specific order is going to incur customs fees, and the Amazon representative emails me back that the order is not going to incur customs fees. Then I make the order, and to my surprise, I do have to pay custom fees. I contact Amazon to ask them to compensate me for the fees, but Amazon now claims that they are not responsible for custom fees. At this point I would be protected by a copy of the email where they claimed that I would incur no customs fees. If I can demonstrate to Amazon that I have proof of their false claims, prior to the purchase, they will be inclined to compensate. If they refuse to compensate, I can (depending on jurisdiction) take my claim to small claims court and present my evidence there. In this case it's unlikely for anyone to actually validate the DKIM signatures, but it does matter whether email is generally considered to be non-repudiable. If you run a campaign to make email repudiable, and make sure people should know email is repudiable, then this email will be less convincing as evidence.
If you run a campaign to make email repudiable, and make sure people should know email is repudiable, then emails will no longer be convincing evidence.