Yes. A person can live without the naturalisation certificate for the rest of his/her life. It is especially true since the introduction of the passport card. I believe there might be some need for the certificate in case of filing N-600 or I-130 for relatives, but I am not sure.If you already has a passport and you can apply for a newone with the old one even expired, does it mean in theory certificate of naturalization is no more necesarily useful so even if you lose it, don't need to pay to get a relacement?