Sirion wont be destroyed, but it can be beaten.
Let's suppose that in the best scenario the south can take from Sirion the Duchy of Oligarch, Krimml, Karbala and maybe, just maybe (what I doubt), Avamar. We will be back to some years ago. And in some years (since I intend to play for more years), you will see Sirion coming back again.
But how you intend to take so much lands and found new realms? Perdan have some recruitment center in elementary schools? Because without new players or a lot of multies, how you will change the entire player base of theses duchies? Atanamir will have to murmur a lot in their ears to convince so much people to betray Sirion, as he did in the south with some leaderships. Here in the north, people know him well enough to avoid his offers. He is hated enough to gather people against him... and in a natural way, every person that hates Perdan will seek for Sirion. That's the way we absorbed members from Fontan after defeat them: giving them reasons to fight Westmoor/Perdan. The same will happens if Caligus disappear.
After spread their players in southern realms, how they will repopulate Sirionite lands? Any expansion or control will be limited by one factor: players. We don't have 120 characters in big realms anymore. People make plans, but they forget to ask Sirion what Sirion thinks about it... so, if you cannot change the mindset, you will never see small realms living for much time around Sirionite former lands. Not because you cannot beat Sirion, but because you cannot exchange the old Sirionites without kick them from the game. I doubt you will see Meristenzio, Ecthelion, Alexander or Erik betraying Sirion and leaving as low traitors. That just to quote some names... then, I'm a little skeptical about these big changes. This should have happened when Erik was Duke of all the conquered regions... and this was attempted, trying to inflate the already huge ego of Erik to make him King. His loyalty is what prevented you from see now the kingdom of Avamar, or any eccentric name to show the predominance of this conqueror
