It makes no sense for us to invest heavily into our attacking options, and focus on wingers, if we are going to switch to a wingerless formation. We currently have 8 attackers, with the inclusion of Keita that would be 9, and even if we are going to loan Kean, we would be back to 8.
I would be all for a midfield of 3, if we were more stacked in the cm department but we aren't. If we bring Matiudi, we have only 6 cms, 2 of those cms being betancur and sturaro. We need to play to our strengths.
Also, as discussed multiple times, a formation is just a bunch of numbers, and because a player is positioned in a certain formation, it does not define their roles within that formation.
Say Allegri started fielding a 4-3-3
Our set starters would be Dybala, and Higuain in attack and one other player from Mandzukic, Pjaca, Cuadrado, Bernadeschi, Douglas Costa, Kean, and possibly Keita.
Same goes for any formation with a midfield of 3. And there is always the talk of Dybala being sacrificed, or him being played out of position. In any formation, unless its with an attack of 2 forwards, Dybala is being played out of position.
Allegri is the coach and he will figure out whats the optimal line up to his disposal, but by saying one formation is better than the other doesn't make sense. The group of players at Allegri's disposal, its very evident where the strengths and weaknesses lie.