Here is another argument. The task which the writer has set himself is to show the vast superiority of Christ’s priesthood to that of Aaron and those who came after him.
How great our confidence in the arrangements of God should be! He has worked out everything perfectly. He has taught mankind with the utmost skill, adapting his lessons to the progress and capacity to understand which the human race has in each age. He has chosen Israel as the vehicle of so many of his lessons, and taught them in a form that is suitable to mankind in the infancy of its understanding. He has set things in proper order so that one truth leads to another, and yet always without revision of his former instruction, because the fulness of truth was fully anticipated in the earlier symbolic lessons. In the last age, the gospel age, he has brought the fulness of his teaching with the coming of the Lord Jesus Christ who did not come to destroy the law or the prophets but to fulfil them. Having grasped the fullness of God’s revelation, we can let go of the earlier preparatory lessons, although we still use them to see the way God has taught mankind, and to study the detail embedded in the earlier lessons which shed much light on the reality that came later in Christ.