To say the New Testament matters and the Old does not is about like saying that your parents matter but your grandparents don’t. It is entirely nonsensical to make that claim. Not only would your parents, and thereby you, not exist without your grandparents, but so much of why your parents are the way they are comes from their parents. That is usually true in a variety of ways even if those parents weren’t around.
We are who we are made to be long before we have any influence or response to the stories that write us. Only then do we get to accept or reject various parts of those stories and begin writing our own. In the same way, the New Testament is what it is precisely because it is born out of the Old Testament story and only then begins to push that story in a new direction.