'Old' is just as relevant now according to 'New'
Matthew 5:17-18
“Do not think that I have come to abolish the Law or the Prophets; I have not come to abolish them but to fulfill them. For truly I tell you, until heaven and earth disappear, not the smallest letter, not the least stroke of a pen, will by any means disappear from the Law until everything is accomplished."
Jews won't accept the 'new' because Xians won't accept the 'old'.
Just as the adversary wants.
Christians accept the old as being for the old covenant and for those in that covenant. If you want to mix the old and the new you end up with new wine in old wine skins.
Matt 9:16
No one sews a patch of unshrunk cloth on an old garment. For the patch will pull away from the garment, and a worse tear will result. 17Neither do men pour new wine into old wineskins. If they do, the skins will burst, the wine will spill, and the wineskins will be ruined. Instead, they pour new wine into new wineskins, and both are preserved.”…
It is not a matter of just fixing up the old covenant so it works better and keeping the 613 Mitzvot. Those in the New Covenant end up knowing the Lord and what He wants.
And of course with the New Covenant comes forgiveness. Moses brought the Law with his covenant and Jesus brought the truth with His covenant.
Jer 31:31 “The days are coming,” declares the Lord,
“when I will make a new covenant
with the people of Israel
and with the people of Judah.
32 It will not be like the covenant
I made with their ancestors
when I took them by the hand
to lead them out of Egypt,
because they broke my covenant,
though I was a husband to them,”
declares the Lord.
33 “This is the covenant I will make with the people of Israel
after that time,” declares the Lord.
“I will put my law in their minds
and write it on their hearts.
I will be their God,
and they will be my people.
34 No longer will they teach their neighbor,
or say to one another, ‘Know the Lord,’
because they will all know me,
from the least of them to the greatest,”
declares the Lord.
“For I will forgive their wickedness
and will remember their sins no more.”