If they are "new" raiders, then where do they come from? Maybe a town got overrun by a wasteland pest and they disbanded? They had a war with the neighbors? Their leader got killed and they fought one another, while destroying their home thus wandering into the wasteland?
The lack of attention to where Raiders come from is a big problem with Fallout 3, which devotes very little attention to the history of the setting. The only answers we ever get are that Ashur is employing Raiders to supply him with both slaves as well as food for his people in Pittsburgh.
No, they are "raider". They also can talk normally and use an array of weapons while creating their own gear.
Also they don't eat people, not most of them. Out of that school you go to in Fo3 with corpses around (again, doesn't mean that they EAT them. Super Mutants do, as dumb that is), we don't know that.
It depends if you think those corpses suspended from chains are just there to look cool or is their version of a larder.
PEOPLE WANT A ROOF. Would you go out the street, strip naked, throw money around and then try to kill the people that go and pick it up? No, right? Then why would 3/4 of the humans in the East Coast "raid"?
Well, most of the Raiders have bases. There's Evergreen Mills, the School, the Suepr Market, and a few other spots throughout the Capital Wasteland.
And if you mean Mongols, they did overtake a civilization. Then mixed with their people culturally and biologically. They went on and use those territories. That's how it works. And even if they themselves "dissapeared", they left an imprint on the neighbout's cultures. And on and on.
The Mongols are notable for the fact they did eventually settle down and mix but history is full of people who just attacked, murdered everyone, and then carried off whatever they could. Tamerlane is particularly notable for the fact his policy of destroying civilizations completely but for skilled artisans he carried back at slaves was effectively the end of high Muslim culture. The Huns also are notable for the fact they were trying to do what Genghis Khan succeeded in doing but after the death of Attila, fell back into their old habits and lost everything.
The BOS could go on and wipe these Raiders. The Ghouls could have killed them as they don't have doctors. They would literally dissapear after a year for, simply put, being all that crazy and just kill each other. Even the New Vegas' Fiends have semblances of leaders and don't infight. And they are the most drugged people in the planet.
The BOS is dying out one soldier at a time due to the fact it's losing its war with the Super Mutants. Why the Super Mutants are in Washington D.C. and pressing them back to the Pentagon. Half of their soldiers have also deserted to join the Outcasts so they're only surviving due to the fact they take Wastelanders on as recruits.
As for eventually disappearing? Probably. I'm confused at the idea that sustainable living is something you think humans are very good at as history has shown otherwise.
The LW arrives at the worst period in Commonwealth history. Without the LW the next year would go like this:
* The BOS gets wiped out by the Super Mutants
* The Enclave torches the Super Mutants
* The Enclave poisons the region/exterminates all ghouls or enslaves the surviving humans.
* The Enclave steals the survivors of Vault 101 to be breeding subjects
* The Raiders die of starvation after they've wiped out Megaton and Rivet City or they move out of the Capital Wasteland to other regions.
* The Outcasts go back to California, probably to die fighting NCR or get killed by Caesar's Legion
* The Pitt fails because there's no one left to rob or enslave.
* The Enclave lives happily ever after in a dead ruin.