The goal of the movie or even the original book was never to make you fear for the future, no matter what people try to read into them. The goal was to be entertaining. The movie ends on a high note, the book ends on a note of irony. I suppose people who found the film depressing would find the book even more so. If you want a less depressing version of almost the exact same film, watch Omega Man. The Will Smith film follows Omega Man much more faithfully than its namesake. There is also a really cool (and very graphic) graphic novel that very faithfully follows the story line of the original novel (which was about loneliness, vampires, and a man seeking to come up with a scientific explaination for why the world's entire population turned into vampires save for him). If the original book had been followed more faithfully as far as the vampire aspect goes and given an R rating, this thing could have gained fame as a very creepy horror film.