> I agree with you here. However, EMail was never designed to do this. Eg if you order products over the internet, how do you know that your opposing party keeps their DKIM key safe?

I get that email and DKIM was never designed for this, it's a side effect. Fingers were not designed for finger print evidence, but it's still nice to have evidence from finger prints, as a side effect from touching things. And the problem of key storage / keys leaking will not disappear even if you change to some different protocol.