[massaman_yams]

I might be reading into this a little, but you seem to be arguing that "Spam is a reasonably solved problem at my scale, therefore spam is a reasonably solved problem at global scale", and those are very different things. One of the key differences is: a small domain like yours is multiple orders of magnitude less attractive of a target, vs. Gmail, and as a result Gmail's filters are subject to constant, high-volume adversarial attacks, and you are not.

SPF and DKIM are pretty explicit in their RFCs that passing authentication isn't a sign the mail is legitimate. The presence of passing auth in a message does change how filters should handle it, but for most larger-scale production filtering systems (not spamassassin) that mostly ends up as "change the weight of certain reputation identifiers in the spam filtering inputs", more or less.