You would be surprised by the number of people who turn down upgrades. I have been dealing with oversold flights recently and picking out appropriate people for op-ups. I would estimate that about 20% of passengers who are offered upgrades decline for whatever reason. Some even get abusive and violent - I remember one lady who tore up her upgraded boarding pass (for an international long-haul flight no less) and threw it in the check-in agent's face screaming "I don't need your ****ing charity b****!!!!".