I feel bad for the many left-out details in both arguments.
There is no safe way to keep a pure-breed going without inbreeding consequences. Once the in-breeding starts bearing phenotypes the 'wagyu' isn't going to taste like 'wagyu' anyways.
I am guessing that the gradient purity of the genotypes of the Australian wagyu beef is due to re-introducing a healthy wild cow back into the genetics of the animal to prevent inbreeding.
Therefore the australian beef would have a more consistent taste to it. While the Japanese version is just a disaster waiting to happen; and only trying to win by appealing to emotion.