Well it could be hard for us fans to understand things. In reality we could loose our front duo in a year. Morata could go back to Real and Tevez propably will play next season and then leave. We don't know but im sure Marotta, Paratici, Nedved knows more. Im not saying Tevez or Morata will leave, but even if Tevez is the only one leaving next season and Morata stays. Im would happily have Dybala, Morata, Cavani in the squad.
Maby our club sees Cavani as the perfect striker for Juventus, and now they see the perfect timing for making a move. We could have all 4 for a season, maby Allegri want to use a tridente Cavani-Tevez-Morata/Dybala. We don't know.
Im sure that if Cavani comes this summer, that the club has a good reason for it.