I don't really get what exactly people blaming Allegri for use Ibra as the focal point want... He got a player who is much better than all the others, so it's his duty to get the best of him and for that Zlatan need to play in a free role. It worked great for us this season, Ibra decided many games.
Yet, he didn't took any other player out of their prefered role to accomodate him. Robinho, Cassano, Prince, Nocerino... All of them were used in their favoured roles and it worked fairly natural.
What's the alternative? Restrict him in the box as a finisher and trust our very creative midfield to get the ball to him? Bench Ibra and use Robinho and Maxi? Build around Robinho, Seedorf or some injured player?
It's not even a question for Allegri tbh.