How do you know that's your bank's signature and not from "h-s-b-c.com"
How does granny know?
Exactly. Any solution you can come up with for baking PGP authentication into email can be applied also to DMARC, so if your primary use case is authenticating reputable businesses why not back the tech that's already seeing adoption by said businesses?
Obviously it'd be great if all digital messages were effectively authenticated using strong cryptography. I imagine at some point we'll get there, probably through a combination of maturing technology and better educated users.
On the other hand there is nothing existing today for telling your MTA to trust this or that domain only. There is no standard so every provider would start by doing their own thing. That missing part is the difficult part, and that is only doable in pgp today.
However it is true that in both cases there should be some UI indication to ask you "do you trust this domain ?"
That has not been my experience signing anything. I cant judge about the math behind the encryption, but the interface side of things has always seemed ... lacking.
Use PGP for this. No, now it is called GPG instead. We thought it was funny. Pass this optional command line argument if you want extra security. Why would we want that as a default?. Enter a passphrase - it is recommended! Except no one uses them. Or they do. Have you installed gpg-agent? Ok so you have a public key now. Before you send stuff to this person you should celebrate a Key ceremony with them and interchange keys physically. Now add your key to your program. Ok so your commits are signed using your key now. If you want to sign a tag, however, you will need to pass this extra flag when creating the tag.
To be fair, that last one is mostly git CLI being consistently inconsistent [1]. But it still another spoke on the wheels.
Encryption needs to be easy in order to be mainstream. Otherwise it will be relegated to the minority which can jump through its interface hoops.
[1] Yes, I am going to be eaten by a hobgoblin any second now http://stevelosh.com/blog/2013/04/git-koans/#the-hobgoblin