I believe that Vieri's loan is structured so that we can call him back mid-season with out too much of a problem. And the Primavera players are available all the time. Plus, Nedved and Camoranesi can play striker. It's not as if we'll run out of strikers and have to forfit the match.
They are better then Chiesa, no doubt. But would they fit in the starting lineup? Even if they did, they would send Salas further down bench. I think a top 3 of Del Piero, Trez and Salas is about as good as you will find anywhere and if CHiesa is 4th, even better. He is a verteran who knows how to get the job done but won't command huge playing time and complain like one of the above.