Diego + Dzeko would have a lot more mobility than Del Piero + Dzeko, plus it would made Dzeko the focal point of our attack, with Del Piero being more "selfish", Trezeguet was happy to let Del Piero do everything and just put the ball in, but Dzeko is different from Trez and would probably want to have the ball a bit more.
Even then, the most important point here is, Del Piero will be 36 in August. He will not have the stamina or the pace to cope with such a heavy load, and unless he stays injury free all season to be able to play each and every single game, the alternative is Iaquinta, the SS. That one, would be a nightmare my friend.