I like to imagine the Legion winning, hence they would be collapsing.
Okay, fair enough. So the Legion are advancing. The NCR are in trouble. If the Gun Runners start a war with the NCR now, the best case scenario for them is that they win. They cut off the head of the NCR. Well done Gun Runners. Now what? The legion are still advancing, who is going to stop them? I doubt many of the NCR veterans you just started a war against are going to help you. No Rangers. No troopers. You watch as the legion swallow up and assimilate more and more of your customers until eventually they reach the Boneyard, enslave your engineers and technicians, and crucify anybody who resists.
They did pretty well without the NCR. And what limits them from creating labour and supplies? It's not like they live in a barren desert. The market isn't prosperous, because having one main business partner isn't the best, mainly because you cannot raise prices or suffer retribution. The NCR can easily pressure the GR to follow their demands. We're not discussing a Fallout 3 world, there would be smaller but civilized societies, and the GR can decide prices at what ever cost they want! It's not like these settlements are going to stop them.
They don't have one main business partner. They have an extremely lucrative partnership with the NCR, but they also sell guns to anybody who rocks up at one of their shops. And sell to caravans dealing with New Canaan and beyond.
The only people they can't sell to are feral raiders, like the Fiends or the White Legs, or people the NCR are actively at war with, like the Legion. The first are hardly a lucrative market compared to what they have already, seeing as they would use the weapons to attack caravans and generally disrupt the economy. The second is pretty much suicide, because the legion has no interest in allowing other factions to exist, as has been shown time and time again.
If they are aiming to break up the NCR and sell to all the different factions that result, they are essentially turning their own homeland into a warzone. What is the sense in that? Maybe they would make more money, but it certainly wouldn't improve their quality of life.
There is absolutely no indication that the Gun Runners are being coerced into selling their weapons to the NCR at an unfair price. And what is this talk of "retribution"? What retribution? The NCR have nothing to gain by attacking their main supplier of guns and ammo, and the Gun Runners have nothing to gain by attacking their biggest customer.
The legion DOES trade with outsiders, hell there's an Arizona merchant in their camp. In fact they make far better trade routes due to the fact that they slaughter bandits and keep the routes safe. The GR wouldn't replace the NCR, they're not that powerful.
Dale Barton is not a faction, he is a trader who works in legion territory. He is no threat to them. You've seen the ending slides for the Great Khans, even when they help the legion, they are assimilated and their identity erased. If the Legion were to win, life would probably be pretty good for a lot of small traders and caravaneers, but I can't see Caesar allowing a large, powerful, independent faction like the Gun Runners to continue existing independently of the Legion.
If the NCR is collapsing (due to Legion pressure) then it's entirely possible they have the ability. Remember that the main percentage of the military is currently in the Mojave Desert. Numbers are no way of fighting theft and industrial sabotage.
If the NCR is collapsing anyway, the gun runners would be better off either straight-up defecting to the Legion, offering their skills in exchange for amnesty, and gambling on receiving Caesar's mercy, or else running like hell into Canada or Mexico. If the NCR as a unified force couldn't stand against the Legion, a collection of disparate tribes and cities certainly won't be able to.
Every single time a quote pyramid is used a baby Gecko dies.
Geckos are evil little bastards. I'm okay with that!