I don't understand this idea that a 21 year old, who's already 32 goals for his club can't become a 20+ season striker. Oh, hang on a minute that's 21 year old Cristiano Ronaldo's stats after a season with Sporting and three seasons with us. At Rashford's age, Harry Kane had scored 21 goals (with 16 of those coming while he was playing in the Championship/League1).
Actually, 6 weeks past his 21st birthday, Marcus has scored 35 for us. Very similar stats to Raheem Sterling at the same age. Gareth Bale was 23 by the time he'd scored that many.
That's not a comparison with Ronaldo, Kane, Bale or even Sterling incidentally, because that would be really unfair because we all have the benefit of hindsight. It's too early to know what happens next. Giggs didn't become a great goalscorer but he was really good to have around, Danny Welbeck didn't make it as a striker either but then Rashford's already scored more goals for us than he did.
He's good for the team right now, better than anyone on the bench - that gets him in the team. I think he's going to be great, but if I'm wrong he's still worth a place in our team, so whatever the oppo pundits think he doesn't have to go anywhere, and we shouldn't let him.