Indirectly this is already the case. Domain names and IP addresses aren't free. If we just immediately trash all email which doesn't have valid SPF + DKIM + re

Indirectly this is already the case. Domain names and IP addresses aren't free. If we just immediately trash all email which doesn't have valid SPF + DKIM + reverse DNS, we place a small upfront payment requirement on sending email. Domains or IPs can be placed on a blocklist if they still send spam, thus making the domain/IP wasted. The problem is that waaaay too many legit senders still don't know how to configure their email servers, so you end up with a boatload of false positives.