There are half a dozen or so The Mist
threads easily found in the archives, including
at least three
dealing primarily with the ending
. Sure, they've been dormant for a while, but it wouldn't have been to difficult to dig up one of them, nor would it have taken much time to read a few of the opinions already offered there...
My own take: it's the perfect
ending. If you want the novella ending, it's already shot and released... in The Birds
. To have kept that ending and aped Hitch's film would've been tolerable, but pretty lame.
Instead, we get a happy ending (humanity seems to win), but not such a happy ending that we can say "it's all right, he's got a new wife, etc." By denying us that catharsis, and the soothing ending of "the little boy will be all right, and, like Harry Potter in the sh***iest epilogue EVER, will one day be totally fine", we're left with the same shock the survivors have.
As for the novella's ambiguous ending... I feel that it's much more suited to the Cold War era of The Birds
/the King novella. Maybe if the movie had been set then, I wouldn't have minded seeing it on screen. But to tack it on now, when it's pretty clear that much of humanity will endure for some time, would have simply struck me as cheap.