I think you mischaracterize Thorin; yes, the Arkenstone was of supreme importance to him, but that doesn't mean he didn't want the gold too.
In fact, this isn't wholly an invention of Peter Jackson; Tolkien discussed the same idea, though less dramatically1:
Long hours in the past days Thorin had spent in the treasury, and the lust of it was heavy on him. Though he had hunted chiefly for the Arkenstone, yet he had an eye for many another wonderful thing that was lying there, about which were wound old memories of the labours and the sorrows of his race.
The Hobbit Chapter 15: "The Gathering of the Clouds"
[C]urb your pride and your greed, or you will fall at the end of whatever path you take, though your hands be full of gold.
Unfinished Tales Part 3: The Third Age Chapter III: "The Quest of Erebor"
Jackson holds that Thorin was overcome by a curse on the gold2, which overrode his good sense and ultimately leads to his downfall; this isn't an uncommon reading, and John Rateliff, author of The History of the Hobbit, is one prominent scholar who supports it:
That Jackson’s Thorin suffers from dragon sickness "is an unusual reading of the it, but one that I advocated for in The History of the Hobbit," Rateliff says. "I was very interested to see that they'd arrived at the same conclusion."
Narratively, this drives one of the major conflicts of the film: other people (Bard and the people of Lake-town, centrally) want Thorin's gold, but Thorin won't give it to them:
Bilbo: [Y]ou made a promise to the people of Lake-town. Is this treasure truly worth more than your honour? Our honour, Thorin, I was also there; I gave my word.
Thorin: For that, I am grateful. It was nobly done, but the treasure does not belong to the people of Lake-town. This gold is ours, and ours alone. By my life, I will not part with a single coin.
The Hobbit: The Battle of the Five Armies (2014)
1 A sentence which rather sums up the entire trilogy, actually
2 There's also basis for this in Tolkien's lore, which I discuss at Is "Dragon Sickness" a figurative or literal curse?