Yeah but that's a stupid point, every player is replaceable at the exception of Messi and Ronaldo, not even De Gea is irreplaceable. Also Mbappé has been put in a position to succeed at Monaco and he was less impressive with PSG, still good but less impressive.
In general, I will never advocate that any player should stay at all costs because a single player whoever he is, isn't superior to a well oiled team, you can always replace an individual by a collective. That being said when you have a talented player, you do everything to exploit his talent, to help him fulfill it and you don't put stupid hurdles on his way.
We have Martial, we don't need to sell him, we don't need to impede him and we don't have a better performer on the left. The last point is the most important, our best performers on the left this season have been the rotation Martial/Rashford and that's where things become ridiculous, some are acting as if the other option was Ronaldo, Hazard or prime Ribéry. It's a below average Alexis Sanchez.
So if you want to get rid of Martial, be my guest but lets not pretend that he has been this season the best performer on the left alongside Rashford and that they are both lambasted by people who apparently haven't realized that the manager's darling was even worse than them.
That idea isn't limited to Martial, it's true for all the players in our team.