If I get email from [email protected], then I now know it's from example.tld. If the mail is spam, I can ask example.tld to block alice, or I can simply blacklist example.tld.
The important part in this is spam prevention, and that if I get an email from @paypal.com, knowing that it's real.
What you're complaining about is another question, but not really relevant. Google always uses @google.com for employees, and if you get an email from that, it's real.
This is about enforcing identification, not verification, security, trust, or anything else.
Is this better than graylist+whitelist? Is it better than content based filtering? I'm not convinced.
Is it better than requiring tls with valid cert for smtp? I can't see how.
Rather than all this mess, I think I'd prefer we start at tls/ssl required for smtp...
It's not like impersonation is dead. When I signed up for the Gmail roll-out, [email protected] was reserved, but you can email me at [email protected] - a poor Norwegian translation equivalent. More importantly, I can email you from that address, and tell you there's a problem with your account, could you please verify that your password is "hunter2", if not email your current password so we can verify that your account is secure (not that I ever have, or will, I got the account for a laugh).
Federated trust (s/mime) or Web of trust/direct trust(gpg) secures email. All this domain pseudo secure dns nonsense... I'm not sure I accept it's better than static analysis.