You might be thinking about it wrong. A site that generates $5K a month is worth way more than some Mommy blog that just started hosting ads.
I assure you, if you wrote a report that targeted your highest earning affiliates, gave them the toss, and replaced them with poor performing newbies you would lose money.
The publisher (ie your website showing ads) isn't the one paying them that money; they aren't losing any recurring revenue by banning them.
Since they're Google AdSense, it's very likely that they have plenty of new signups to show those ads on.