It can be permanently erased. But only by intent. There is absolutely no way to lose this range of data. Suggesting such an event is criminal. Someone will break the wall of silence at some point. Contractors do not like to go to jail, but the IRS is the great bogeyman.
I hope Issa will start jailing IT people, as now it is clear that at least the records retention laws were not obeyed. And they can't exactly plead the fifth on it, because Issa has an admission that they were erased. Once people start sitting in jail, even if they are lowly IT techs that didn't have anything to do with it except follow orders, people will start talking.