One problem is that with GMail taking over so much of the world the spammers have become highly focused on defeating their filters. Worse, they seem to be slowly but continually increasing their success rate, all while non-spam is ever more frequently choked out by false positives.

The end condition of this race is only spammers will be able to send mail to Google, no legitimate users will have the time or budget to figure out how to get past all of the blocks.