Its a team sport and the manager decides what is best for the team.
Initially Rashford and Martial were basically alternating, but Rashford has since pulled clearly ahead.
Lingard does not really play in the same position as Martial, and even if he is limited in talent, he makes up for it in effort and tries to do the right thing. Martial plays like his mind is elsewhere, playing in his own world. He gets the ball and runs at defenders until he loses the ball and then looks at the sky as if expecting some angel will come down and help him recover it.
I see Lingard running with the ball and losing it way WAY more than Martial. And like I said Martial has double the output, you can't brush that under the carpet. Also while Rashford alternated the LW initially with Martial, both Mkhi and Lingard got quite a few games in that position especially since Ibra's absences started (with his red card first, then his injury after his return).
Like you said, it's a team sport and the manager decides what's best for the team. I think the fact we're finishing the season 6th with the lowest goals scored than all other top 6 teams (and Bournemouth), is a clear indication for me that manager has made plenty of wrong choices, this one being one of them.
Feel free to disagree. But that's my opinion.