When you get down to it, Simon and Shinji share the same character archetype (as well as Naota and Noriko). It's *because* Simon had Kamina (as well as Nia, but initially Kamina) that he became who he was. Given a Kamina or a Coach or whatnot from an early enough age, Shinji would be the same as them.
Which, when you get down to it, is the point. Shinji's failure as a human being is his success as a character. Shinji is the flip side of the coin, the failure where Noriko and Simon were the success. And the show does a great job at expressing that. Why do we know that? Because, 13 years later, everyone *still* hates Shinji.