#2 The competition will make him a better player

Anthony Martial now finds himself in a spot where he has to consistently push himself to stake a claim in the starting XI week in and week out. While actual minutes on the pitch go a long way towards the development of a youngster, intense competition on the training ground will most definitely make him a better player.
Marcus Rashford wasn't even the best striker in the youth team when he broke into Manchester United's first team. But since getting there, he has improved immensely and that's because he had Zlatan Ibrahimovic to groom him.
Martial could do the same with Alexis Sanchez to improve himself. Sanchez's doggedness and sheer will to win the ball back are traits that the Frenchman could most definitely imbibe.
He could just ship off to a club where he can fit into comfortably and do what he does but at the end of the day, wouldn't he become a better player if he stays at United and learns from the best?