You dont always need to play in the striker role to show the striker instinct. Many times I have observed there is some tempting cross into the box and Marcus just doesnot have that instinct to attack the ball. No wonder he was behind Fletcher in youth setup for a long time. Other attributes like holding up the ball, link up play and passing in and around the box, is also lacking. The best position for him to develop is wing forward, the central position is just too much for him.
Regarding 20 year old Kane, I think its fair to say only 1% of all highly rated youngsters actually develop into top player like Kane. And Kane was more of a late bloomer. If you think a good player at 20 will go on to become a top top player at 25, then we should be keeping Macheda, Ashley fletcher, Januzaj, Memphis, Wilson, Keane. They just were not going to be top players.
I get your point that if you dont give chance, you never know but the reality is its getting tougher season by season to groom younger players in any top clubs including United. The stake is too high, the risk is too high. The way we are grooming him, giving him time at wide forward position is the best way to develop him.