DKIM is certificates, so I'm guessing you're talking about sender certificates?

How would that help? Spammers can get certificates too. Maybe it cuts down on some of the misconfigured http email senders, maybe, but not enough to matter. Scam sites run https these days.

You can't use like age of activity of the cert to help because a) things get compromised, b) you need to rotate your certs frequently anyway.

Being able to tie together letters and senders, knowing who sent what, would help.

It wouldn't help to fully trust, nothing would, it's a human problem, it would help to trust more.

If the email is DKIM signed, it's expected that the sender was authorized to send the message.

Any wide spread certificate program will just have the email address as the identity, and it will be authorized by establishing control of the email (just like the majority of certificates used for https are domain control only, no organizational verification, not that organizational verification means much anyway). Anyway, identity is hard; there are many people with my name, including a Pulitzer winning author.

Shouldn't the authority (or key) used to sign those certs be long lived? The certificates themselves should be rotated frequently, bit not the key used to sign them.

There's only S/MIME, TLS and BIMI+VMC that use actual certificates.

DKIM does not and DKIM keys should be rotated once in a while, but few do.