> What is being suggested is that if a domain has a proven history of not sending spam
That is merely because of using DKIM! Everyone can create a 'history of not sending spam', including spammers.
With your solution someone can rent a subnet, warm up a bunch of domains, but then blast spam from the entire subnet without immidiately affecting the entire set of spam domains.
What you're proposing wouldn't allow trusting even signed mail from an already suspicious subnet. Be the domain with or without reputation, so are the spammers'.
> Not blocking the domain does not increase the amount of spam that gets through because the domain is not sending spam.
They don't know that before they deliver your mail to inboxes and someone does or doesn't mark it as spam.
You keep missing the point. Nobody is suggesting that mail be allowed merely because a domain is using DKIM.
What is being suggested is that if a domain has a proven history of not sending spam then blocking the domain's mail just because there are spammers in the same subnet results in blocking non-spam. Not blocking the domain does not increase the amount of spam that gets through because the domain is not sending spam.
Where DKIM comes in is that it allows receiving sites to distinguish between mail that is really from domain X and mail that is forged to appear to come from X. The latter can safely be automatically classified as spam. The former can be used to build an accurate history for the domain to determine if it sends spam or not.