Sorry, for being confusing. The Catalan example was not coined why the game in mind, but with the Byzantines/Spain not suddenly feeling as a different entity because a (sizable) part of their entity took a different road.
Regarding the game: we will have to see how exactly the beginning of a new age looks. We know that cities change - they are either destroyed or demoted to towns (as one golden Age ability is that "cities remain cities"). We know that most buildings become useless shells. I imagine - but this is just how I put information together from the screen that told us what you can "purchase" with legacy points, no hard facts - that all your military units are dead. So, it seems that when you start a new age, there was a traumatic event, i.e., you did not survive, at least not in a form that is good enough to just continue.
How this works when you are successful in the crisis before in the game? I don't know. I also don't know how successful you can actually be in it and what happens, aside from you eventually having to put in 6 cards with penalties. The crisis has three stages - maybe you will fail it anyway and its just about "how far can I survive and get these additional legacy points or this golden age?" We'll see. For me, it's definitely the most interesting open question about the game.