Why the Na'vi might live happily ever after:
This planet, I understand, is in the Alpha Centauri system.
That means it will be 4 years before Earth even hears of the rebellion (2158).
About 5.5 years before the surviving Earthmen get back to explain what happened (2159.5)
Now let's assume that there are Earthlings who are going to take a strong "no war for unobtanium" stance, along with truly thinking conservatives who understand that the Na'vi were, in fact, defending their private property (which is the real message of this movie; don't steal shit that ain't yours, monkey boy) - the Company is going to have to deal with this somehow, along with the fact that nobody really likes losers. So funding for the Great Pandora Invasion Fleet is going to be somewhat difficult to come by, even if a Bush is President-CEO of Earth. So let's say, four years minimum to get all this put together, even if it's possible at all. Huge expenses involved here.
So really, is the Company going to glass Pandora, really? Are Earthlings really going to tolerate genocide? I doubt it. Right now, you have to have O2 masks to survive on Pandora; are you really willing to have your people work in radiation suits, too?
So assuming the glassing Pandora option is out, President George W. Bush V decides to invade. That means the mighty Earthling army will have to fight a bush war at the end of a 5.5 year long one way trip supply line, against a whole planet where the natives know the land better than you and can make their weapons right there...unlike you, General Halliburton, who has to have every single bullet and gallon of chopper fuel, every man and gun and helicopter, shipped to you over 4 light years of interstellar space.
Costly. VERY costly. And in the meantime the Na'vi are going to have at least a decade to develop better weapons, tactics and, also, to dig in.