I think wherever he goes he'll play better and be more consistent then what he'll do here next season. That's why I can't blame him too much for wanting to leave, and why it might be for the best ultimately. Sometimes players need a move to make that step up instead of a gradual increase in playing time,and sometimes they just need the right system to suit them to bring the best out of them. It's no secret that Mourinho's system doesn't get the best out of attackers in general, let alone the type of player Martial is. A wide forward like him needs to play in an attacking system, where he has a high starting position, can make runs in behind or drift inwards when he gets the ball to get goals, but generally always have his space to attack. Look at Mo Salah, people often use him as an example to criticize Mourinho with. Salah for Mourinho even right now would be pretty ineffective. Completely conflicting styles (de bruyne is different). Martial under Klopp or Pep, in a very attacking system like they have where they take loads of risks going forward and always commit bodies in attack. Martial has plenty of space to improve himself of course, and isn't without his faults, but a better system would get so much more out of him and I think he'll show that wherever he goes. Even if it is to Atletico, since his role in a defensive side would be replacing Griezmann basically so would have a roaming attacking role.
Of course, we would hope that he could develop here, but it's unlikely, and we'll always be rotating people without anyone fully becoming what they want to be or what they can be because of shared time. Even if you ignore Sanchez being here. Our long term future only ever held one of Rashford or Martial in the side. With Sanchez here especially, there's only room for one, but the thing that changed is there's less time to share now. It sucks, but it is how it is because of how Mourinho uses Lukaku. If Sir Alex was here, I have no doubt that we'd be playing a form of 4-4-2 where 3 of lukaku, martial, rashford and Sanchez are always on the pitch. And the 4th rotating constantly amongst those 3 and subbing any of them off, and all 4 happy and playing. Mourinho though never subs lukaku, never rests him, and almost never changes the system to accommodate 2 strikers. So there's no room for all of them.