Alen, the point in developing youngster in most cases is having world-class or at least extremely good players around them. Today Juventus doesn't have them and therefore it is key to buy already skilled players since only in the future they will help to develop others. Many may not agree, but I believe that is way Marchisio never realized his full potential.
As for Sorensen, he is talented, but from whom he could learn? Motta? I don't think so. Playing experience? Maybe, but I don't want to see Juventus suffer for another couple of years only to see Sorensen becoming great and being sold.