I know there are successful movies that have done it but not very many animated, full length ones.
And the two examples you used bed were both of 'historical' characters with stories that are largely known. Not only that but they were technically 'robbers' so they weren't pure good characters so the people out there that don't like to see a movie ending with the main characters all dying can justify it (not saying that is how everyone thinks but there is a wide spectrum of moviegoers to try to please).
I'm not saying it hasn't been done, or it can't be done. I'm just trying to guess based on current trends. I can't wait till we hear more about the story. I'm pretty sure I will see it when it first comes out (unless it gets super terrible reviews, then I'll wait till it goes to the dollar theatre near me

*edit* Also this idea come out of cultural ideas about story telling. Most of what I am saying is for the American market. From what I have heard/been taught, the European story lines for movies aren't as obsessed with the 'happy ending' as it is here in the U.S.