The important part of what I said above is that the spam would not arrive in any inbox. The temptation (and ability) to look through spam is the current problem

The important part of what I said above is that the spam would not arrive in any inbox. The temptation (and ability) to look through spam is the current problem, and would be eliminated. If some servers are already doing that, then they are doing the right thing - except, the message body of a spam should not be bounced. The extra traffic on the bounce is worth expending, since it is necessary to keep spam out of sight. If they can't get their message through, they lose the incentive to send spam. That's all that matters.