This angle has also been done to death now ("Twilight, "Aaah! Zombies!", "My Best Friend is a Vampire", various episodes of Buffy/Angel, etc...)
I don't agree with this conclusion. While there are indeed a huge number of stories about fighting the self, that doesn't make every one of these potential stories about that. You're ignoring an entire category -- external threats are easily separated from internal threats (although many good stories include both).
However, I find stories that treat an external threat as just a metaphor for the internal struggle to be pretentious and, often, kinda boring. How many times do we have to watch "man is the real monster of them all!"? We don't know that! It's a presumptuous claim to make, lacking in imagination.
I also don't see this as a valid conclusion. That's like that patent office guy from back in the 1800's who said "Everything that could possibly be invented, has been invented."
I think what often happens is this: any set of subjects/topics/ideas can be split in various different categories, depending on the criteria of the categorizor. But then, if this set of categories becomes popular, people start to treat that set of categories as non-arbitrary and then as absolute. The very notion that there's a different way to slice it is forgotten.
I see this all the time in the realm of symbolism. Storytellers going waaay back interpret a set of symbols a certain way ("a circle is unity!", "cloven hooves == bad!", etc...) and then everyone latches on to those as if they were a fundamental part of the universe/psychology/fairyland/whatever and no one ever bothers to see their own symbols, or to interpret the world in a different way.
We keep going back to the same sources, forgetting that once upon a time those sources did not exist and were invented by people just like you and me.