While it would have conveyed the same message, I think a sarcastic "Yep" would have been at odds with the tone of the scene (and Buffy's response). Buffy's feeling wistful and at an end of her journey. I like the way she phrased her response because it isn't obvious. It makes you think. She's contemplative. Over five years of Slaying and trying to show people that 'girls' can be strong, and people are still questioning her about it. And now she's about to save the world (hopefully) and this random kid is telling her she's 'just' a girl?

I still love it. It's subtle, which I adore. It makes you think. It's not hitting you over the head with a sledgehammer to drive home what Buffy means (I abhor episodes that use the sledgehammer method). I wouldn't want it changed.