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.