If you could change ONE thing in The Hobbit triology

[Aaron]

Originally Posted by Nerwen

Losing credibility is always a potential problem with recurring villains or ones who get a lot of screentime, but as you say there are ways around it.

I do think the length of the films to be once again part of the problem- I guess they felt if they restricted Smaug to just his appearances in the book, it would be- relative to the running time- almost a blink-and-you-miss-him situation.

The problem with Smaug in the book is that his death does not serve the narrative - but the symbolism Tolkien wanted to convey.
Had he been trying to write a simple adventure story, then yes, the heroic Dwarves would kill him in fair battle, or perhaps even Bilbo would find his courage and strike the killing blow himself?
But who defeats him? The very people he was gloating about destroying. With the help of a thrush. The Dragon, a destroyer of both man and nature, is undone by his own sins coming back to haunt him.

Just as Thorin's party is, when the Orcs arrive to avenge the Great Goblin, and, in Bolg's case, Azog.

A book can take these kinds of huge risks, and trust the audience to perceive the moral message in it all. But films, especially big blockbusters, often seem to be designed as disposable, forgettable cash-grabs.
In that regard, the films were very, very successful.