Yes Pizarro has a better pass than De Rossi has, but that's all. Pizarro could never do what De Rossi can. Pizarro can play so freely, because De Rossi allows him too. De Rossi, by being so complete, can adapt and cover and play reserved. Ability-wise he is still up there, you can see it in every touch. Remember, De Rossi started as a high central midfielder.
Roma's midfield would fall about without De Rossi. The team would lose all its function and structure.
You are blaming De Rossi for what you're praising Pizarro for - one good season. Pizarro last season was his finest, by far. Before that, he was mistake-prone and often a ghost. Like I said, Aquilani was his superiour by far. Pizarro the best playmaker since udine days? He was a ghost at Inter as well..