Your missing the point of the series, it was never about being happy. And it shouldn't end with EVERYONE being happy. Everyone's character coming full circle, yes that should've happened, but happiness was never part of the Joss equation. The whole point is "life goes on", Joss himself knows that life is not full of happy endings, it's full of struggles and the series finale is going to symbolize that. And it's not like the ending was depressing and they didn't lose everything, I mean yeah Sunnydale was destroyed, but it was on top of the Hellmouth and I'm sure Buffy knew that was going to be part of the battling The First deal. And I would say that at least Willow got somewhat a happy ending, she became all goddess like for a couple minutes and was the only one who left the series in a relationship. But yeah, not meant to be overly happy, that's not Joss's vision.