Just as in the 1970s, a Golden Age of modern cinema, the ending to Sunshine is not necessarily the happiest. Its simply the most appropriate ending. Just look at Midnight Cowboy. No way it should have ended on a happy note. And since happiness is such a subjective concept, it might be in everyone's best interest if the crew of the Icarus II doesn't survive.

Afterall, the film discusses the Sun. The creator and source of life. Mortality comes to the fore in any discussion of creation. The effective visuals of light and darkness are an allegory for the message of the film. And the messiah status attained by the crew when they give their lives to ensure the continued existence of humanity is a better ending then a survivor walking away.

It all boils down to what you take with you into a movie, just like the director William Friedkin said about The Exorcist back in the 70s.