I don't see how, in your preferred replacement email architecture, a provider would be able to avoid policing their users to prevent spam in the way that you complain is so burdensome.
To begin with, mail could only enter such a system through port 587 or through a rogue operator signing an email peering agreement. In either case, there is a bilateral contract involved so that it is clear whose customer is doing wrong, and therefore who is responsible for policing it. It's a different model in which email traffic follows a chain of bilateral agreements from the sender to the recipient. At each link in the chain, a provider can block traffic if it does not conform to the peering agreement (or service agreement for end users). Today, an anonymous spammer can obfuscate the source of their email in a way that an average user can't figure out who to complain to. In a hierarchical email peering system, only a rogue operator could do that, and by nature of the system, they can't really be totally anonymous. After all they have to sign a peering agreement with someone. --Michael Dillon