Factions have died by being integrated into other factions, new ones have been born from nothing and from other factions. A good example of the latter is how the Bleeders formed - primarily, but not only - from the ranks of Ghost Eyes and Hearts.
Actually getting in a faction is easy. Getting out of one might be a different thing, because one becomes attached to the advantages related to being a faction member (if one attains enough independence, that is) despite the chance of death. Curiosity propels anything in the Caedaren perspective. The process is simple as hell. It's walking to any barracks or whatnot of whatever faction one wants to join, talking to the quartermaster or commanding officer and signing a form if one is deemed fit enough: there may well be tests. There might be an evaluation period. But it is still childishly easy to at least attempt to join a faction. In the same manner, retirement is a possibility at any point, but the retirée is closely watched. The factions dislike their secrets being told to other factions.
(See also, freewarrior.)