About the studio forced ending...
I like downer endings (the ending in The Mist is a great example, although the pacing was too rushed and because of that, it made a great movie into only a good one) but I dont think that this movie would work with that approach. The reason why I think the "studio" ending works is not just because the efforts by the hero are not in vain. The "studio" ending inadvertently follows the spirit of the novel, in that humans will fight back, and might even win.
P.S. - I just saw the movie for the first time, and I liked it a lot (though there were some problems). I actually felt suspense towards the end, which for me almost never happens with "oldschool" cinema.
"Fighting for peace is like screwing for virginity." - George Carlin