Of course the real answer to the question, “How should The Desolation of Smaug have ended?” is “With the end of the goddamn story because there’s no way The Hobbit needed to be three, three-hour movies.”
Buuut what the hell, let’s play along with the How It Should Have Ended folks, who imagine a Middle Earth where luck dragons exist and everyone tries to resolve their differences with Smaug in a friendly way. It doesn’t go so well…
Okay, let me explain the eagles thing — they don’t use them all the time because they have to climb a tree to call them, and sometimes the tree is toooo tall to scale. That’s the explanation I came up with as a kid, and I think it still holds up pretty well.