I have no problem with top clubs buying young players and developing them. The difference is that a youngster was our most expensive signing by £20 million (hency why i said large amounts of money). It is not sustainable to spend so much on youngsters, especially when there are many youngster on a similar level to Martial. Madrid have Carvajal, Vasquez, Nacho in and around the 1st team, Barcelona have just gone through a period where many of their youth products were in the first team, the same can be said about Bayern Munich (Schweinsteiger, Muller, Kroos).
When you spend £50 million on a youngster he has a larger expectation to produce than a player that cost nothing. I do not think it is right because players develop at their own speed, but football is a business and that is the reality of balancing the books. And nowhere have i said that Martial shouldnt be given time to develop, just that he will be given less time because that is the what makes the most sense business wise.
I want Martial to develop and flourish at United, but his price tag means he will be given less time by the club. I would give him as much time as needed within reason, but I am a fan not running the club's finances.