Let's please take black and white out of the equation. You do understand that the comics you love was founded on that very principle, that white kids only could like and admire white male heroes that where just like them.
Your basis for this?
The fact that basically all superheroes created from the 1930s to the 1970s (and pretty much all the major ones to this day) were white (and overwhelmingly male)?
Not all children's heroes need to be of their race (non-white kids have been indeed idealizing Superman since he first appeared), but some should. Things like an absence of powerful and cool pop culture role models of one's own race/gender/whatever have a demonstrable effect.
You're correct about the Roman Empire, incidentally; its collapse was caused by longstanding internal problems (and large numbers of invading hordes that were, for the matter, pagan). Not to mention that ignores the survival of the Eastern Roman/Byzantine Empire, which was every bit (even more, perhaps) Christian than the western one. Of course, I rather doubt that that graph is meant to be taken seriously.