Those priests appointed under the law could not have remained in office forever. It would have been inappropriate for that to happen.
He is our Saviour. What Levite was ever in a position like that? Here is the Levitical priesthood, with its splendour and its vestments, its liturgy, its rituals, and its dignity. But by contrast, Christ is so much greater, eternal, the King of righteousness and King of peace. You can see it all in this wonderful chapter. If you just read it through, especially in the King James Version, and you read it calmly and you bear these things in mind, then every time you read it you will see more clearly, something special about Christ, about our privileges, about his position in this great prophecy of David.