Ask yourself: when is the last time Western civilization has stumbled across a pool of massively valuable resources, and just walked away because we met resistance?
After losing billions of dollars and being humiliated by a bunch of giant blue Ewoks, is it more likely that the head of the corporation will see the error of his ways, or will he invest the rest of his assets into burning the surface of Pandora with orbital missiles before sending in an assault fleet the size of Colorado? Did the Europeans give up on the New World's gold the first time one of their settlers took a tomahawk to the head?
Remember, the humans only lost to the Na'vi because they were poorly equipped, fighting with machine guns and fragile aircraft that couldn't even stand up to the local wildlife. They were lacking the future-generation cruise missiles, stealth bombers and who knows what other kind of weapons of mass destruction they've invented in the intervening 150 years. But they'll have them when they come back.
Hell, we're pretty sure this thing is arrow-proof.
And they'll have the permission to use them, too. After all, the aliens killed a bunch of humans. Hell, they'll probably get a government bailout for their losses.
If He'd Just Stayed Away:
True, the Na'vi would have been forced to give up their huge tree (the one sitting on top of the minerals) and that process would still have turned ugly. But after the tree fell, without Sully there to whip them into a fighting frenzy they presumably would have acknowledged it was time to simply find some other huge tree to live in (or maybe settling for several smaller ones). They'd have been forced to figure out a way to coexist with the humans... saving their species in the process.