Well, one way to approach it would be to say if they want you to give up your seat, then move you up a class to give you more room. Ultimately it's the airlines fault for over booking, they are the ones who decide how big they make the seat, so they get to deal with the problems that arise from those choices.

As it goes, while some routes get oversold, quite a few do not. And other passengers will only know that you purchased that seat if they have to ask for volunteer bumps. So most people will assume that was just a seat that didn't get sold or the passenger never made it.