Truth be told, I don't see why it is that Martial is such a topic of discussion given that other positions are much more troubling in terms of a long-term fit. Obviously, we don't have someone of a high calibre to play on the right side of attack right now, and it could also be argued that improving CM, CDM, CB, and LB to an elite level are all more pressing concerns right now than improving at ST.
I think you can break down any squad of players into three groups: those who are obviously not good enough and definitely need replacing, those who are possibly good enough but we aren't sure, and those who are definitely good enough and are sure-fire long-term starters. Obviously, you want to always get rid of the first group of players in all situations; in the second case, you only get rid of those who are potentially good if there is no deadwood in the squad and if the squad is by and large made up of elite players. I find it hard to believe that this current United side is mostly made up of elite players and would find it even harder to believe that there is no deadwood in the squad. Therefore, it makes very little sense to get rid of Martial any time soon. If Martial was to remain in the unsure category and the squad around him improved, the case for getting rid makes a lot more sense, but that is not the case right now so I don't see any reason why we should get rid.
Even at that, I also find it strange as to what people value in a striker. People complain about Martial not being a proper CF but do you need a proper CF to succeed in the modern game? A False 9 isn't a proper CF but that doesn't mean you can't succeed by playing one. Similarly, I always feel that one of the key distinguishing aspects of a CF is good hold-up play. In that instance, is someone who consistently makes good runs behind the defence but can't hold the ball up well a useful player? Of course they are so it's not really that important whether or not a player is a proper CF. Again, I'm talking more in general terms here as people are often critical of Martial for not making runs in behind, but the point I want to make is just because he doesn't play up front in a similar style to a CF doesn't mean he can't play up front.
Moreover, I think we have to re-assess how we define the quality of attacking players. Football as a game is a low-scoring game so any individual goal is very significant in a way in which scoring a three-pointer in basketball is not. Therefore, imagine a striker who has maybe 40 actions in a game. Imagine then that about only 20% of those actions are successful. Then imagine that out of those successful actions, 12.5% lead to a goal. If these figures took place in a game, a striker would average either a goal or an assist per game. Over the course of a 38-game season, this would be an outstanding record given that the most productive of last year's top PL scorers finished on 22 goals and 10 assists. So obviously a striker can have a monumental impact whilst not being very efficient. Ultimately, if a striker frustrates most of the time but has a good scoring rate, I find it hard to to understand the argument that they have no place in the team. Looking at ProSoccer Reference, only 7 players have a better Non-Penalty Goals Per 90 Ratio than Martial (0.53) in the PL this season, namely Vardy (0.61), Aubameyang (0.62), Aguero (0.9), Ings (0.68), Mané (0.61), Abraham (0.6), and Calvert-Lewin (0.61). Given such productivity, I find it hard to see why people seem to think Martial has not been delivering the goods. If anything, our ability to score goals has likely been hampered by having zero reliable goalscorers outside of Martial and Rashford for most of the season (Greenwood hasn't played enough so I'm leaving him out) and having not had either Bruno or Pogba playing for most of the season. Therefore, I don't really see how it is that Martial is the one holding the team back and is in need of replacing.