It's not that the old was bad. It served a good purpose, for it helped restrain sin and promote godliness. But it was weak, temporary, and imperfect. And God never promised Aaron or anyone that the old covenant and the old priesthood would be anything but temporary in nature. No priest could serve forever.
Death has a way of removing people from office.
Death has a way of removing people from the role they play in our lives. A sick person has a family doctor, familiar with his case and his entire medical history, having provided his medical care since birth. Then the doctor dies and now the patient must start all over with a new doctor. A business man has a lawyer who knows his affairs and has represented him since the day he began his business. Then the lawyer dies and now the business man must start all over with a new lawyer.