Does Allegri deserve credit for integrating youngsters? Imo not really, no.
His hand was forced, indeed. If we had the 2015 midfield, he'd have never given a chance to Fagioli, Miretti, Barrenechea or Iling. Someone from the upper echelons decided that we can't afford to pay the wages of Arthur and Zakaria, or not to offer a new contract to Bernardeschi, but imo it still had to be approved by Allegri. I don't think that they didn't even ask him when they decided to get rid of these three, and then to get rid of McKennie in the winter.
And if these four didn't leave, the youngsters wouldn't have been given the chance to play so much, or to play at all.
To summarize, even if he isn't someone who will integrate youngsters at the expense of better players, he is surely the one who is playing a lot of youngsters this year, and he was almost certainly the one who gave a green light for the loans of some more experienced midfielders, relying on the youngsters to successfully replace them.
If he didn't trust young players as some claim, he'd have never allowed the above to happen.