Depending on what the carrier rate is, DY can decrease it. At some point, as was pointed out, the decrease would be so minimal it wouldn’t be noticeable. Here’s the math. Let’s start out assuming it’s a 50/50 split between carriers and non-carriers. Now, there are 4 possible couples, with Y for carrier, N for non-carrier: YY, YN, NY, NN. YY would be rejected by DY. Therefore, 1/3 of couples would have no carriers, while 2/3 would have 1 carrier. Their children should be 50/50 between carriers and non-carriers. That gives us 1/3 of the next generation as carriers, and 2/3 as non-carriers.
Now, do the same math over again, but only 1/3 starting off as carriers instead of 1/2. The percentage of carriers will drop every generation. For the next generation, the percentage of carriers drops from 33% to 25%. The next generation drops to 20%.