It happened again and again the pope lived really long, seemingly the pope won't die if there is no preferati.
I think that's a good feature. The main focus of the game is the Catholic factions, hence being 13 of them. If the Pope disappeared, a factor of the Catholic factions has. So the Pope has to stay, even after annhiliated.
Saying that, after taking Rome I have never seen where they move to. How long does it take them? Do they just 'acquire' a settlement or still take it by force? Or does a Catholic faction give a settlement to them?