I don't reckon he was offered the #10 just yet. The club probably don't want to seem fickle, handing it out willy nilly. The #10 was vacant a year after Del Piero.
Dybala screams #10 though. He is so reminiscent of Del Piero in so many ways beyond his style of play. It's inevitable that he'll be offered it and I doubt very much that he would refuse. It's wishful thinking in this day and age but he could stay at Juve forever if we're lucky. He doesn't seem like the type to chase the $$$ but any non-Italian's attachment to the club is rightly questionable.