Okay, goals win games, so far we agree. So your solution is to bench Zlatan, who scored an obscene amount of goals last season, in fact the most productive season in his career.
Your point being that Rashford is a great goalscorer, and that's his greatest asset, when in fact he's so much more. His appetite for taking on defenders from every position in the final third, his intelligent runs and directness are so, so beneficial for all the other attackers, it was clear as day when he came on vs Hull, and he completely changed the dynamic of the game. Add to that his brilliant ability to pick out his team mates in tight areas in and around the box (has he ever missed a backheel pass when playing for us at senior level?), and you should start to get a picture of a player that can be influential in all three forward positions. You know, without taking out the player who is by far our best goal scorer (Zlatan).
I would argue that Zlatan would benefit Marcus more than Rooney, did you per chance see when Zlatan put Rooney through on goal with a 30 meter pass with his backheel? He draws CBs towards him, and that would be a major advantage for Marcus, wherever he plays across the front three. While Rooney has more of a tendency to drop deeper and deeper, leaving players isolated.
Marcus can easily play right or left wing/forward and contribute with goals, assists and penetration from out wide. I wouldn't want him behind Zlatan in the middle, because his speed would not be utilised to the fullest there.