Honestly I see very little emigration happening. There will be those with the money to do so, but lets be honest. People with money in the game are usually a duke or prince in a prominent house anyways. So that leaves those who have spent some money but not allot, is it likely they'll move to another kingdom? Whats the incentive? Inless a large amount of people move, with sufficient power then it means little in the scope of territory gain. Because lets be honest, the people who decide the outcome of wars/city captures are the whales, and the smaller whales that follow them. So unless those individuals uproot and move to a new faction, without any direct inventive at first mind you, then nothing will change.
The incentives would have to be forming a new house, and gaining access to cities again as a prince/duke (mind you that the king system mitigates some of these issues, as you can appoint those individuals to one of the new ranks), but their will be marked drop in overall territory control. Since their not bringing the 20-80 house members with them(at least not likely) so the power and ability to capture and then KEEP a city will still basically be the same in the gaining kingdom.
So no, I dont see emigration really changing anything let alone fixing the current and ongoing issue that plagues any server older then a month.
Which is population. Can it be solved? Sure, find a way to protect lower end players, I wont go into the million and one ways to do this. They can pick one or any and go with it. Otherwise servers will continue to bleed people into newer servers, while either stagnating into a stalemate as defined by the algorithm that seems to define the state of all the older servers.
Whale + Player count + time coverage = winning kingdom
Whales can be mitigated, so can time coverage, nothing solves a lack of players to sit in an acquired city and act as a wall.