What if Google got so good at filtering out spam that the spammers gave up and decided to pay for Gmail text ads to get to their audiences? When you receive spam on Gmail, you are encouraged to report the spam as spam. It disappears from your inbox and Google gets a report for its spam database.
With all that spam information centralized, I would think Google could get pretty good at filtering out the spam. Users, especially those who like free email accounts, might be happy about exchanging (hopefully tasteful) sidebar text ads for (1) the outrageous torrent of spam we get these days and (2) banner or pop-up Ads. Google could get very rich indeed.