You assume that the Brotherhood has been doing nothing for the past 80 years. We can't assume that the have beend oing nothing, as they very wel could have been making contact with various other military remnants, searching out other caches and depots. The Enclave had the capability of the oil rig and what amounts to the Secret Service, as well as the ancestors of a marine detail? Power armor is great, but its effectiveness compared to normal power armor and combat armor was not as vastly overpowering as the Enclave would try to make it? Ignoring the FEV, the Brotherhood might very well have a good chance, should it break its isolationa nd dole out technology to trusted wastelanders. I call to mind that NCR seemed pretty damn powerful, what with gauss rifles, functional combat armor, and a pretty dedicated citizenry could hold out quite a bit against the Enclave. Recall that its territory basically encompasses much of the old Fallout 1 area, and theoretically could have access to the Vault in Necropolis, the Sierra Army Depot, the Mutant Base and various other locales. How would the Enclave replace its own troops, if they are unwilling to work with "mutants"? They quite literally would have to make do with the Navarro base detachment and the defenders on the oil rig. Do they have the fortitude to conquer AND hold an area encompassing all over California to wipe out the BOS? Hardly. The BOS in turn could appeal to some sort of messianic belief, rearming the wastelanders with some well needed equipment. If they have enough ammo to blow it on miniguns, surely they have other caches elsewhere of weaponry? I mean, mini energy cells and micro fusion cells were pretty plentiful, to be available on the markets for the Chosen One to purchase. That means that the defenders of the city probably had stocked up on such advanced tech, and didn't need the remnants.