The writer of the Book of Hebrews sought to show that Jesus surpasses even Aaron as a high priest, because it is well known that Aaron and all his sons died, but the high priesthood of Jesus is based upon the order of Melchizedek, whom the Bible suddenly introduces to us in Genesis 14 and then we do not hear anything of his death.
Hebrews 7:8 says,
“Here mortal men receive tithes, but there he receives them, of whom it is witnessed that he lives.”
The levitical priests received tithes, but they all died. Melchizedek certainly died, but his kind of priesthood did not then shift to another man. The kind of priesthood he had lives on in the high priesthood of Jesus Christ, who continues to live.
Abraham was greater than Levi, but Melchizedek was greater than Abraham, and Jesus is greater than Melchizedek. Nothing can surpass what we have in Christ.