Spoilers below...
I think, with Smaug not being in the film for not much at all, he shouldn't have died off at the end. That's what they've done. Since the antagonists had to share screen time, they couldn't get enough footage for both to kill one off at the end.
They want to show more on the characters, delve deeper into them so fans don't get upset because they didn't see much of that evil character, or that he was in one film.
If Smaug had a lot of screen time in the second film, let's say about an hour and a half to two hours, he could've been killed off. Providing that they had you anticipated for the third film and defeat of Sauron, as in giving him enough screen time alongside Smaug to make you want to see what happens to that character.
Neither evil characters got enough screen time, and so they want to continue them on in the next film so you get to see more of them.
The reason they've left all those cliffhangers ie. Bard being apprehended, Smaug flying away, Legolas chasing Bolg and Gandalf trapped is to want you anticipate the next part. So when it comes out, you're going to buy tickets to see what happens next after the previous film. This way, they're happy with their money. Let's admit that some people would not see the next part if Smaug died, seems as he is the main antagonist. Fans however, would likely see the third film though if this happens.
As frustrating as the cliff hangers are, it's best for both. Us, the fans and the producers.
Let's stop here before I ramble on too much