I've said this in a couple threads, he is the best striker for this team. That doesn't mean we can't upgrade on him and I think mentally he is a bit weak. But he is the only striker who can actually hold up the ball, dribble, pass, link up etc. We need a player like him for our wide attackers and attacking mids to play off. Both ronaldo and cavani provide none of this.
Whilst Martial has no complaints being replaced, I think he does have the right to feel aggrieved that he has been replaced by both cavani and ronaldo who just arnt as good as him in regards to the striker role for this team.