This is a reasonable position. The thing is, on form, he is capable of displacing any of the current starters. That keeps the current starters on their toes. That in itself has value. Not just being able to bring him on in games. Dan James and co have no chance of displacing Rashford.
Talk of loaning him out or selling him is frankly stupid if you're not bringing a player of equal or better standing.
Bringing in Sancho and getting rid of Martial is a sideways move squad wise.
Cavani, Sancho, Martial, Greenwood, Rashford are all top players on their day and should be competing to start. That's what top clubs do.