Surely you understand that the main function the High Priest is to stand in the "gap" between God and his people?
Deuteronomy 5:1-7 KJV
1. And Moses called all Israel, and said unto them, Hear, O Israel, the statutes and judgments which I speak in your ears this day, that ye may learn them, and keep, and do them.
2. The Lord our God made a covenant with us in Horeb.
3. The Lord made not this covenant with our fathers, but with us, even us, who are all of us here alive this day.
4. The Lord talked with you face to face in the mount out of the midst of the fire,
5. (I stood between the Lord and you at that time, to shew you the word of the Lord: for ye were afraid by reason of the fire, and went not up into the mount;) saying,
6. I am the Lord thy God, which brought thee out of the land of Egypt, from the house of bondage.
7. Thou shalt have none other gods before me.
Likewise who anointed the Altar and the Mishkan-Tabernacle with all of its vessels? (Exodus 40). Who anointed Aaron and his sons? (Exodus 40). Who sprinkled both the Sefer and the people with the blood of the Covenant? (Exodus 24:7-8, Hebrews 9:19). Who is the brother of Aaron? As far as the sons of men by whose hand was it that the Most High brought up the children of Israel out of Egypt?
Deuteronomy 18:13-19 KJV
13. Thou shalt be perfect with the Lord thy God.
14. For these nations, which thou shalt possess, hearkened unto observers of times, and unto diviners: but as for thee, the Lord thy God hath not suffered thee so to do.
15. The Lord thy God will raise up unto thee a Prophet from the midst of thee, of thy brethren, like unto me; unto him ye shall hearken;
16. According to all that thou desiredst of the Lord thy God in Horeb in the day of the assembly, saying, Let me not hear again the voice of the Lord my God, neither let me see this great fire any more, that I die not.
17. And the Lord said unto me, They have well spoken that which they have spoken.
18. I will raise them up a Prophet from among their brethren, like unto thee, and will put my words in his mouth; and he shall speak unto them all that I shall command him.
19. And it shall come to pass, that whosoever will not hearken unto my words which he shall speak in my name, I will require it of him.
The only covenant which was "disannulled" is the same which was prophesied in Isaiah 28 where it is called a "covenant with death". And why is it called a covenant with death? Because that is how the High Priesthood ruled the people, from the death of one and election of the next, to the death of the next and election of another; by succession. These are the same that ruled the people from Jerusalem as the passage states:
Isaiah 28:14-18 KJV
14. Wherefore hear the word of the Lord, ye scornful men, that rule this people which is in Jerusalem.
15. Because ye have said, We have made a covenant with death, [for it was not given by an oath in Torah] and with hell are we at agreement; when the overflowing scourge shall pass through, it shall not come unto us: for we have made lies our refuge, and under falsehood have we hid ourselves:
16. Therefore thus saith the Lord God, Behold, I lay in Zion for a foundation a stone, a tried stone, a precious corner stone, a sure foundation: he that believeth shall not make haste. [clearly Messiah Yeshua]
17. Judgment also will I lay to the line, and righteousness to the plummet: and the hail shall sweep away the refuge of lies, and the waters shall overflow the hiding place.
18. And your covenant with death shall be disannulled, and your agreement with hell shall not stand; when the overflowing scourge shall pass through, then ye shall be trodden down by it.
Hebrews 7:15-24 KJV
15. And it is yet far more evident: for that after the similitude of Melchisedec there ariseth another priest,
16. Who is made, not after the law of a carnal commandment, but after the power of an endless life.
17. For he testifieth, Thou art a priest for ever after the order of Melchisedec.
18. For there is verily a disannulling of the commandment going before for the weakness and unprofitableness thereof.
19. For the law made nothing perfect, but the bringing in of a better hope did; by the which we draw nigh unto God.
20. And inasmuch as not without an oath he was made priest:
21. (For those priests were made without an oath; but this with an oath by him that said unto him, The Lord sware and will not repent, Thou art a priest for ever after the order of Melchisedec:)
22. By so much was Jesus made a surety of a better testament.
23. And they truly were many priests, because they were not suffered to continue by reason of death:
24. But this man, because he continueth ever, hath an unchangeable priesthood.
The old covenant of the High Priesthood was "disannulled" because of the weakness of the ordinances due to the FLESH. The Priesthood reigned from the one man to the next through the death of the former and election of the next in line. Therefore the ordinaces were not perfect yet not because of the Law but because of the temporary nature of the body of the man which kept "waxing old, dying, and decaying." And not only that but because those men were not perfect men in the sight of God. Yeshua is now perfected and eternal at the right hand of the Father.
Old Testament-Covenant Saints ~ Body of Moses
New Testament-Covenant Saints ~ Body of Messiah