> "... small non-spam senders would congregate" Congratulations, you've invented the email service provider. Most small non-spam senders don't want to spend the time figuring out IP reputation, DKIM, or SPF already, so they pay someone to do it for them. What makes netblock reputation different?

Reputation isn't public because then spammers game it and you get worse filtering outcomes. ISPs learned this the hard way.