> 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.
The original email spec doesn't provide any security against forgeries. The "sent from" field in email is about as secure as the "sent from" field in physical letters. The only reason why laypersons consider email to be non-repudiable is because of additional protocols like SPF and DKIM that were implemented after the original spec. Without these protocols email would be considered repudiable, which OP considers to be a preferrable outcome.
> And that commerce existed before emails?
Yes, and? I'm not claiming that all commerce would come to a halt immediately if this campaign for email repudiability was successful. Of course commerce would continue to exist. But the world would be worse off, not better. There would be slightly more disputes, and dishonest parties would increase their chances of defrauding honest parties.
> You don't need DKIM to solve the issues you've pointed out.
Are you alluding to hypothetical alternative protocols for authenticating contracts? If you can make the world move off from email, that's great! Email is horrible! But if you can't make people move away from email, you won't make the world a better place by making email less secure.
> Again: How many disputes like that have been resolved with DKIM?
How many? As in, you expect me to have statistics on it? Are we pretending that when people resolve disputes, they mark their disputes in some kind of global database that we can query for statistics? You're not making any sense.