Amauri is not bad. In fact he is a very good player and more complete than trezeguet. But trezeguet can score. And when a striker scores you don't want more. He has the killer instinct and the extraordinary finishing. He knows every time where he should be. Ok he gets old and that meanes less speed and less stamina, but also means more experience. If some time before he needed 2 chances to score, now he needs just half. I don't want to say the same things all the time and being boring, all of us know what trezeguet is. I hope Ciro knows it too. They have played in the same team for 5 years...
Ciro will know about every Juve player better than any of us do on here.
I agree with the basis of your reasoning about David's goalscoring prowess and experience.
However, even though I'm as big a fan of his talent as the next man (or woman), there comes a time when a club and even a player needs to refresh. Invariably, this is best achieved by a change of circumstances. Usually it means moving on and a career is revived in new surroundings. A new city, new club, new colleagues and so on.
I think David has reached that time in his career. Just like Vieira did at Arsenal when he left in the Summer of 2005 after a long, successful spell in a great squad. It's a scenario that affects many players.
I know many on here would be happy to see him stay. I know that he will probably have a much better relationship with the new coach. We will just have to wait and see what happens.
The process of undergoing private discussions with each and every player will soon be underway. Whatever the outcome , Juventus will come out of it well. And David will too.