This one off cross will vanish as it gets further into the gene pool with pure dallies.

Yes, that's the point. But as it is absorbed into the existing gene pool it will add to that pool the coding for healthy urethic acid production. Unless shunned by breeders that coding will spread widely within the pool and resolve an otherwise intractable health problem.

Of course it means that pool will no longer be 'pure' by your measure but since all breeds stem from mixed foundation stock your measure of purity seems arbitrary and, given the inherent health problems of closed gene pools, without merit.