I understand your point, but Bale, Asensio, James and Isco are not in Mariano's way since none of those players ever play as a striker like Mariano.
At Real, Ronaldo, Benzema and Morata play as strikers and Mariano is 23 (24 in august) and is built like a tank so it's easy for Zidane to give him a chance.
But your right in that there is a way for Brahim to get a chance, but as I see it, I think it's unlikely and that he'll go on loan. I'm starting to get the feeling Pep will demand almost all the youngsters to have loan experience just based on the way he talks about youth. Brahim could go to Malaga or Girona next season which is my guess as to where he'll be.
I think that since Sancho is an inverted left winger, and we have none of those, has a better chance because there is less competition. That's just how I think it'll go down, obviously I'd be delighted if Brahim did get some games.