The thing about "Manifest Destiny" is that now it has some pretty bad connotations with the whole "Indian Relocation" thing. The term "Destiny" in there refers to the belief that it was European-descended settler's "destiny" to own all that land as a God-given right. And yeah, Graves does take what he wants, which is what they were doing, but they probably just don't want to use those words.
So it's usually kind of something people don't use in modern Western stuff because of that issue. It's a pretty interesting topic. /historydork
Plus, I kinda like New Destiny because, like Helmight said above, Burning Tides changed Graves immensely. He lost his grudge and his lust for revenge (WHOOOOOOOO), which means he really is trying to find something else to live for. And it's a new gun.