It would. But I think the real threat is that man-made diamonds will likely become impossible to distinguish from natural, even with the expensive, specialty equipment that debeers has created.

At that point, natural and man-made will reach a price parity... Far below the current cost of natural diamonds. Because there will be no way of knowing where your diamond comes from.

Well, at that point they can create a "guaranteed provenance" diamond with papers like a show dog or a Mickey Mantle autographed baseball. That will help shut out even other mined diamonds vendors since all the marketing will be for the DeBeers branded diamond.

Though that could backfire. “This diamond was mined in a certified conflict zone.”