>> Email can be secured and authenticated with GPG and perhaps more people should consider doing this.
> Doesn't work unless you teach people about security, which is difficult.
Simple, transparent security is hard. But this is again a UI problem, not a failing of email.
Think about this for a second: let's say Google decided that it was going to transparently PGP-sign all GMail users' emails. Now GMail knows that any message with a from address that ends in @gmail.com but without a valid PGP signature is fraudulent.
Now let's say Yahoo wants to adopt this too. So we add some syntax to SPF that advertises, "any mail coming from this domain must be PGP signed." And another field that points to a public key server. So then all other SMTP servers (or even end-user mail clients, if they want), can auto-reject mail based on this criterion.
Sure, this leaves out some details, like people sending mail from an @gmail.com address using their own SMTP server via a thick mail client that doesn't support PGP (or requires complicated setup to do it properly, like most do). But it's a (hopefully) interesting idea that might work with some modifications.
This has nothing to do with email per se, nothing about its successes or failings. It's just building an easier-to-use distributed authentication system that mail recipients can use.