As you know, there are many legitimate needs to publish naked pictures of your body to the entire internet, but most people want to keep them private. Defaults matter.
What makes this hard is that email is responsible for too much crap. No single user interface should carry:
1. Party invites
2. Private messages to your spouse, therapist, pastor, etc.
3. Marketing messages
4. Password recovery requests
5. Financial transactions
We've just shoved them all into email because it's there.
I agree that e-mail is horrible, and I would prefer if contracts (like purchase receipts) moved off e-mail to something else, but that's tangential to the point. If Google started to publish and rotate DKIM keys, the world wouldn't suddenly move away from email.
In the real world it doesn't matter which cryptographic protocols are theoretically available for use. What matters is which protocols everyone else is using. For example, in the case of receipts for purchases on the web, literally everyone is using email. You will not be able to get amazon to sign a receipt with gnupg.
If you want to embark on a path of convincing the world to move away from email, that's great, good for you. Just don't pretend like removing non-repudiation from e-mails is a quest on that path. It's not.
Which is why, when email providers can do very simple things that improve privacy for all their members, even though email is irretrievably insecure, they should do so, as harm mitigation. Your mail spool will eventually get owned up. The least they can do is make it deniable.
What makes this hard is that email is responsible for too much crap. No single user interface should carry:
1. Party invites
2. Private messages to your spouse, therapist, pastor, etc.
3. Marketing messages
4. Password recovery requests
5. Financial transactions
We've just shoved them all into email because it's there.