That's what you think, but in reality he is not useless and can be useful.
Let's not compare keeper situation with the outfield position. There is a reason why City has solid squad depth in attack but not in goalkeeper. Rashford or Cavani is not Lewa though, Lewa is like Bruno, Ronaldo, Messi won't be dropped. Again, Pogba can still play in double pivot against team that park the bus. And whether you like it or no, Cavani cannot be forced to play week in week out, that's fact and that fact is a risk that we decided to take. If he's dropped then the likely choice to replace him will be Rashford which happened vs Aston Villa and previous games.
Yes they will be happy sharing, why not? If you are not happy then pack your bag like Lukaku.
Just imagine play Cavani up front with Rashford on the left and Pogba in double pivot against Crystal Palace in our last day of the league match to determine our league title and then play Pogba on the left in UCL final and Rashford as a striker since Cavani needs rest.