I wouldn't make a big deal about the sharingan as a decendant of byakugan. For one thing, Kakashi mentioned it out of passing, as a myth. Even if its true, it would be a trait that likely evolved very gradually throughout the history of the clan.

And let's not forget one key factor in evolution, if newer genetic mutations end up remaining in the gene pool, then they must provide some sort of superior adaptation for the creature to its environment.