Instead of trying to convince readers who have already made the decision to unsubscribe from your emails, to change their mind, why not instead present some alternative marketing options they may not have thought about on your unsubscribe page?
Maybe they would prefer to receive updates via Facebook or even Twitter instead of messages in their inbox. They may still like your company, but email is not the best way to reach them in the future.
Sure, some readers will stick with you forever, but some will start a conversation with you, then decide they no longer feel like chatting. This is OK. So, why not give them the opportunity to love you in some other capacity in the future?