If they had done something different with the Enclave, like they actually being a remnant of a remnant, imagine like Autumm's dad and Autumm escaped the oil rig with the President AI, and after traveling to the capital escaping the persecution of the NCR they stumble upon a rather large military base full of old equipment like power armor, vertibirds and weapons, Food supplies and the like.
After a few years of living in the base, isolated but well fed Autumm jr. starts to get weird ideas about restoring the Enclave, probably thanks to the President AI so he starts hiring mercenaries by giving them the equipment in exchange for work, they take over a few vaults acquiring needed scientific personel to prepare for a takeover of the Capital, but they know they have limited resources, so they mostly keep on the downlow, they may have the base and equipment, but their vertibirds barely have any fuel, the armors they have are either in poor condition or are all rather old models, while they could try and force everyone under their flag with force a battle of such proportions would drain them rather quickly, after learning of this Project that seeks to purify the water of the Wasteland they decide the best course of action is to take over it so the people of the region have no other choice but to join them. The AI may have it's own shortsighted plans for the Projectm but Autumm actually looks forward to use the Purifier to gain power rather than genocide.
The Capital BOS could have similar intentions but different approaches, so the concept of Helping either group would have it's own set of pros and cons. And then there would be a reason for all those people that want to have a "join the enclave" option to want it.