On when a faction dies: here's my idea.
When there's options: what happened will be a function of chance, relations, and power.
If a faction has an adult male heir- they become a sovereign, and the faction stays in the game.
If a faction has egalatarian, and an adult female hair, they became a sovereign. The husband will join the faction if there is one, kids have a chance to.
If a faction has an adult female heir, and is not egalitarian, then the heir's husband becomes ruler. There's a chance the faction remains independent or joins the husband's faction. If the faction stays independent, the wife becomes the sovereign. If the faction does not, then the cities join the husband's faction.
If a faction has a child heir, then it joins the surviving spouse's faction.
If the faction has a child heir, and it's not of a surviving faction- then go down the line. Faction may destruct, chances increase the further down the line you go.
If the faction has no heir, or the spouse is not of a faction- the faction destructs. Cities might surrender to the conqueror, based on charisma. If cities dissolve, refugee camps and adventurers will spawn from this.
This is probably overcomplicated and imperfect, but an idea of the type of depth I'd want in this system.