I think Lingard is fantastic - however I see Lingard getting the ball in positions where Martial has better capacity to take on the players in front of him.
These positions that Lingard get in to his due to his fantastic positionining - but saying that Martial would only need to to position himself few times a game to be absolutely clinical with his chances.
Lingard is better for a defensive alternative to our attack - possibly even being the highest forward player that puts pressure. Lingard as an attacker is a player that I call a 'pivot' player; he either makes short passes to interchange & pass the ball to attackers around him or he tends to position himself to the point where the attackers around him have less defenders to deal with. Lingard is a pivot that helps the players around him even if he doesn't particularly rack up numbers.
However when we don't need it it's better to have a player like martial drop deep centrally to receive the ball and start our attacks. He can get the numbers, he can come deep, can make short passes & can take the Central defenders on with ease.
The Firminh0' goal where he takes 2 defenders on in to open territory is p*ss easy for a player like martial more so than any other player. In my eyes players who have to defend against martial are always in a 50/50 position of either taking him on or leaving him be. On the wings martial can be allowed to go past the fullback but still is in a position where he can't properly attack - ie being out too wide; but centrally & deep - martial has the ability to come at the CB's with pace and the ball at his feet; having the ability to beat them with a single piece of skill before being one on one with the GK.
You make a good point but as of right now I think where Martial is being played provides balance to our attack in the team.
Martial provide that width which we need because every single ones of our attackers love going inward and playing in the middle, even Martial himself love coming inside and dropping deep as well but he still provide that width.
Martial is also our best dribbler in attack and good 1 on 1 against fullbacks. While it's true Martial playing centrally would be even more beneficial for him(I'm of the same opinion) but the team still need someone to provide some sort of natural width which Martial does unlike everyone else. Him starting wide this providing width then coming inside has been working just fine and it's very integral to our new way of playing.
This is why I'm confident the left wing spot will be his own while the striker potion and especially the right are still up for grab. Though lately with Rashford form one would think he's nailed on that striker role. Martial and Rash should have those 2 spots nailed down for now.
In a way you could say Martial is sacrificed a bit in playing on that left wing role(because like you I believe he's even more deadly centrally and that would also get him a lot closer to goal than on the wing) but him on the left has given us such a good balance and he's good there as well anyways, that it makes sense to keep him there.