Why have none of these top sides ever tried to sign him then?
As I said, he is elite in one or two areas of the game. Give him space to run in behind and he will punish teams because he's lightning quick and clinical, he also strikes the ball equally well off either foot. Ask him to contribute in possession though or create something against teams who sit back and limit his space then more often than not he'll have very little effect on the game and you'll barely notice he's even playing. Rashford at 22 already matched(albeit with a few penalties) Son's best output in a season so on what basis can you definitively claim that he'll never get anywhere near Son's level? He's already shown he can, he hasn't been able to do it as consistently as what Son is now doing in his late 20s but that comes with age, Son wasn't this good five yeas ago either. Rashford may never get there but if he doesn't it won't be because he lacks the ability, it'll be because he lacked the mentality to make the most of his talent.
I don't know what's going on with Rashford, maybe he will drop off the face of the Earth in the same way that Dele Alli has but for anyone who can remember beyond last week, he is still a player with all the attributes to be amongst the upper echelon of wide forwards.