Well yea the depth problem has become an issue in recent years indeed. But i'm not 100% sure who are the young probable stars playing domestic cricket ATM, so i cant be certain.
That was because AUS learnt from their mistakes in previous series & picked a better bowling attack - a 4-man pace attack. Plus the
key factor Mitchell Johnson from nowhere began to swing the ball at always 95 mph (according the the SA speed guns
) which shocked the Saffies as much as it did AUS.
By the capetown test, when the probably adjusted themselves to Johnson it was too late AUS had already won. Although compared to the first two test in Jo'Burg & Durban - Capetown was a road, so that also nuetralized Johnson's wing as well.
So overall it was no fault of SA.