Alen, you do have some good points, but replacing a player like Trez or Camo could prove to be impossible. Especially when you consider that we'll more than likely have a very limited transfer budget.
Let's single out Trez for a moment. Selling him won't get you any real money because of his age / current contract, but we would indeed save some money on wages.
On the other hand, you could also opt to sell Amauri. You'd save money on his (also significant wages) & he would still fetch a decent transfer sum. Not as much as we paid for him, but still.
Overall, I'd think that the finances for both possibilities are pretty much the same.
So you either keep an injury prone but proven world class striker or a decent striker who's rarely injured.
Personally, I'd opt to keep Trezeguet, for the simple reason that a player like Amauri is easier to replace. Add to that the fact that Amauri's value has been decreasing & will continue to decrease even further (regardless of form, he's getting older). Which means that, once we do part ways with him, we can kiss a nice transfer sum goodbye.