You are correct for certain definitions of 'stainless steel.' However as other comments have alluded to, some grades of 'stainless steel' will rust so slowly that they will survive orders of magnitude longer than recorded history. For example, some "Hyper-duplex" stainless steels are designed to resist corrosion in seawater that is above boiling point. This steel is probably not economical for building construction, but if you wanted it to last for millenia, it may be the optimal choice.