Not sure if that is a valid argument against death penalty.
It can be re-worded.
Advocating imprisonment implies being fine with some innocent people being imprisoned.
Does that mean prisons are wrong because there MIGHT be a chance an innocent person gets into jail for - maybe - 30 years until it becomes clear was was innocent? An innocent person might get sent to jail and DIE before a new technique (like DNA tests) proves his innocence.
You cannot avoid imprisoning people (I am fairly dovish on this issue and think that prisons are too often hell, a place where people change for the worse and not for the better ... but you obviously do wanna imprison rapists and murderers and not just force them to do community work or pay a fine.) but you can avoid executing them.