The high priest would die and then it would be necessary to find a successor to become the high priest. And then he would grow old and die and they would need another. And another and another. There is only Jesus. There is no replacement for Jesus.
None is needed.
When He died upon the cross, He arose from the dead and ever lives to perform His priestly duties. For the Law appoints men as high priests who are weak, but the word of the oath, which came after the Law, appoints a Son, made perfect forever. The final point of comparison between the Levitical Priesthood versus the priesthood of Christ is a moral comparison. We have a high priest who is holy, innocent, and undefiled. Though He was identified with our sins upon the cross, He is separated from sinners and exalted above the heavens.