Except for mailing lists. And email forwarding. And a lot of other stuff you take for granted. Users might not explicitly know about the technical details of how a lot of things work, but that doesn't mean they don't expect the resulting behavior.
It's fashionable, these days, to put authentication and authorization in the transport layer. It's not a natural law, though. As an example, consider s/mime or pgp message signing. MDAs could validate signatures and discard messages that are not correct. This is an example way to solve the auth problems you describe without breaking existing mailing lists, since you'd be validating the content instead of the envelope.
Just food for thought. SMTP isn't as broken as you think, and replacing it isn't necessary to solve today's problems.
You can't ask regular users to run pgp themselves. This is a prime example of the sort of technical details that a user shouldn't be asked to even understand or be aware it exists.
Lipstick on a pig yes, optional, no. If you want to have your e-mail delivered to Gmail you'll need them. And since Gmail is 50%+ of all e-mail that means it's pretty much mandatory.
Maybe for you. But considering I use gmail I block it all and never notice any problems