In my opinion, the West Coast BoS really do need to adapt to survive.
But seriously though, what is the Brotherhood's purpose? What are their goals and plans? All they have ever seemed to do is act as glorified vultures, what do they do with all of the military tech that they salvage, just store it somewhere in a big vault?
After FO3, it depends on the region. In the west, they're glorified scavengers and raiders that are perfectly willing to sit back and do nothing with their power as whole communities die.
In the midwest, the Brotherhood is pretty much as massive, sprawling extortion racket.
In the east...the BoS is wildly different. Lyons turned the Brotherhood from an isolationist paramilitary organization into a solid core of warriors and scholars thats actually
establishing relations with the rest of the populace. They provide the technology and protection for communities to build, and a structure for a general order to be built upon. As the super mutant threat wanes, and with the Enclave crushed, Lyons' Brotherhood will probably put more and more into building and maintaining lines of trade and communication, running patrols to subdue raiders and Outcasts. Eventually they would return to being a smallish corps of elite warrior scholars, but ones that are beloved heroes, an order that children dream to join. A classic "Knightly Order" that exists to teach and protect.
If, of course, leaders as forward thinking as Lyons stay in charge.