I don't think Hiro's trying to prove anything. He's a legit nasuverse fan, and I think he wanted to take the story in this direction because the he likes the dark chuuni parts of Fate and wanted to write something like that. How well it works is up to each individual. But I don't see Hiroyama as trying to prove anything or insecure of the loli fanservice and light hearted start of the story
I mean, if he was trying to remove that stigma or something, would he have made the final ascensions he did for Illya and Kuro?