mootinator makes a good point. But I can imagine a public that, while not interested in unfortunate endings per sé, might be interested in "neutral" endings, namely the skeptic community. Real life is not supposed to suck, neither is it supposed to be good to us, it's rather "indifferent" to our fortunes, although the word "indifferent" is a bit too antropomorphic to my taste here.
I think it is a real challenge to write a story that is appealing to humans, yet is not taking an either all is good or all is bad view. Most interesting stories are interesting because they appeal to our prejudices in some smart way, and yeah, a public will want to see a nice ending, another public will crave for an apocalypse... but the public for "things happen" is certainly not so broad. Just write down what a typical week of your life is and try to sell it in bookform, fail guaranteed. Yet, I think there's something there. There must be a way to combine the two forms. After all, many history books are just that. If history books for the great public or books inspired on historical facts were just as dry as the academic stuff, nobody would read them. They somehow manage to be interesting by weaving a story out of the otherwise boring succession of chance and necessity.
There are many classics that are "morality neutral" in this way. For instance Dostoievski, although he was a fervent but doubting christian, writes in a fairly a neutral style. I've never felt that he pushed his own ideas onto me. Yet, his books are all about ideas. There are no good or bad endings in his stories, for some characters, things turn out well, for others things don't. And the distribution of good and bad outcomes is not correlated to the goodness of the characters in any way. Of course, Dostoievski is quite high level literature, not accessible to children, but I was just looking for an example.