I’m not arguing for the point that the poster you are replying to made, but what they posted is not illogical.
His point is only illogical if you believe that an infinite amount of time has progressed since the Fall of Man. How do you come to the conclusion that some decay == total decay over a finite amount of time?
That’s like my saying that Entropy dictates that there should be zero energy in the Universe even though it is 13 Billion+ years old!
Because you are starting with a faulty breeding pair, to begin with, and your gene-pool is EXTREMELY limited: two individuals.