You have to have a working mail server attached to a domain to be able to send mail... that's the big part. Right now, email can more or less come to anywhere from anywhere as anyone. There are extensions for signing connections, tls, etc... but in general SMTP at it's core is pretty open and there have been efforts to close this.
It would simply close the loop and push the burden of the messages onto the sender's system mostly.
And yes, you can decide from the envelope, and a higher chance of envelope validity.
SPF doesn't prove that... I can send through SendGrid with an SPF record, doesn't mean I've got a server configured to receive mail... for that matter, it actually makes it so you HAVE to be responsible for the mail system and cannot outsource sending separately from receiving. Again, shifting the burden enough to where other measures of dealing with bad actors are more effective.
It would simply close the loop and push the burden of the messages onto the sender's system mostly.
And yes, you can decide from the envelope, and a higher chance of envelope validity.