Consider Joe Montana. If I did my calculating right, he averaged under 19 tds and about 2930 yards a season. Respectable, but not great. Nobody is saying that he benefited solely from Walsh's system. There is something that Brady brings that no one else does today. Part of it is leadership, but there is something else. He will be put in a class by himself.
If I had more time, I could devolpe this point more, but I have to study for my AP Gov. exam. Brian will probably pick it apart.