My first assumption would be that with SBS selling the Monarch packages a certain level of starting stability of the kingdoms has been implied. In regards to drafting armies and such and going on wars of expansion as a way to (1) reduce your population and (2) inject wealth into your domain and keep your people happy we don't even have the information to make any assumptions. Some tribes have much more combative cultures but none seem to have a real tradition of organized soldiery for the state. I am surprised by others being surprised that biomes are heavily weighted towards specific tribal populations when we have known for a while that tribes had fairly specific biome preferences.

I hope that the mechanics for managing your NPC populace also include factors like war weariness and really needing actual supply lines for large scale combat meaning that this isn't something that you could do week one of the game but would be an action taken by a fairly organized lord who has spent some time preparing their people for this.

In short I think that any organized migration of the masses will be something players are unable to undertake early on and specifically regarding the movement of people from high population centers with needs unmet to population centers where there is room for them will occur in a rather disorganized every mann for themselves manner.

The lord who wants to build that massive war machine from the advantaged position of a very large populace I will posit would be well advised to really build up their domain as a place people WANT to stay first and THEN establish a standing army and THEN send their people to war.

... There is little or no distinction between a mass migration and an invasion, particularly at low tech levels. One of the primary ways that conquerors secure territory is by establishing colonies absorbing, displacing, or exterminating the native culture and replacing it with their own.

I think this is the core "mechanic", if you will, upon which valid fears of intentional conquest-by-migration are based. There are some assumptions involved that may not be valid depending on what the detailed mechanics of migration will be in CoE.

1. Migrants will function like Meeples when they settle in another domain, extending the effective political control of the lord(s) of their former domain.

I don't think this is a valid assumption. An obvious rhetorical question is, if their former domain was so good, why did they leave? Less obviously, migrants will bring their cultural affiliations with them, and may not originally have any cultural affinity with the existing residents of their new lands -- indeed quite the opposite is typical. This is something that a domain lord would have to deal with. If the domain lord ties indigenous culture too closely to his politics, then the likelihood of conflict escalates, and the likelihood of dynastic survival decreases.

Similarly, I don't see any evidence that the game mechanics will support aggressive lords moving loyal NPC populations into neighboring territories (like meeples). If this comes to be the case, then it violates the principle that NPCs have "minds of their own", with their own individual hierarchy of needs. While it is certainly possible for PCs affiliated with a lord to advance the lord's interests by "infiltrating" neighboring domains, the same is possible for the lord whose domain is open to "infiltration". This is simply a kind of PvP that the game is being designed to not only allow but foster. IMO, "infiltrating" PC settlers will have their hands full merely surviving, much less prospering enough to become a fifth column or staging ground for political or military takeover. In the meantime, the lord of the targeted domain has local resources to contain the threat, especially if that lord has been active and vigilant.

2. Low tech level is a key determinant in the effectiveness of conquest-by-migration.

I think the difference in tech levels between groups is a more significant determinant. Using the Yamnaya culture as an example, the critical factor evidently enabling their replacement of Stone Age Europeans was their mastery of chariot technology, rendering them militarily superior to the Europeans who preceded them. Similar cases can be made for cultural "genocide" in the Americas, albeit abetted by a massive disease gradient, whereas there was less of a technological advantage by Europeans over the major Asian civilizations.

That being said, the technological differences among starting Elyrian tribes is not nearly as pronounced as the historical examples. Among the tribes, technological superiority is evidently being used as a balancing factor for physical inferiority, but no tribe has any obvious unique advantage with the same degree of disparity as chariots versus militia infantry.

... The Romans, Saxons, Danes, and Normans all invaded England and each one was fought bitterly by the native inhabitants. The ethnic similarities between these groups, of which there were many, didn't seem to stop them from murdering each other in large numbers. Their cultural, linguistic, and political differences were enough.

I think this is a picture painted with too broad a brush. Saxons did not genetically replace all prior Celtic inhabitants of England. There were distinct regional borders dividing the two ethnicities, according to the recent research I've read. Similarly, Danes did not entirely displace Saxons, nor did Normans replace either Saxons or Danes. Normans did supplant prior Saxon political structures and entities, but retained what was useful to them. The Norman invasion was not a movement of peoples but a movement of aristocrats and soldiers, who were seeking already developed lands to rule over and enjoy the fruits of rather than to personally farm themselves.

... My expectation from all the information that I had available before D&SS indicated to me that the biomes would be more or less homogeneous. I don't know where you got any contrary expectations from. In some instances I feel like people are retroactively reinterpreting information to justify the outcome, but I digress. I would have also preferred to have homogeneous biomes at game launch regardless because it would have given the players a cleaner slate to play with at start, I think that's a fair thing to want (to have wanted anyway).

I agree that tribally-homogenous biomes was a reasonable expectation at one time in development. However, I detected clues indicating that complete homogeneity was unlikely even before Kickstarter. The degree of intermingling evident at the start of "real" D&SS was a little surprising to me, but I had seen hints that the world-generation algorithms were at least producing intermingling -- in particular one developer comment that Yoru were being generated in Kingdom 1. Albeit, I tend to try to suppress my expectations, rather than build up on insufficient evidence and wishful thinking what is likely to disappoint me, so I'm not a good subjective judge on what reasonable expectations are.

... Flatly, making arguments about what will happen, or what naturally would happen, requires observation and data, and you can't make claims based on the current state of the game world because that is something the devs just made up unilaterally. It indicates nothing about the political, social, or cultural cohesion of the kingdoms, and it indicates nothing about how these situations may develop going forward. It certainly cannot be used to predict the outcomes of migrations that happen in play. It's just the way things are to begin with for important, and arbitrary, plot/game reasons and that's it!

I can make predictions, but there's no reason for anyone to take them seriously or even consider them without corroborating evidence.

We don't know exactly how socially and culturally cohesive the larger political entities will be. Given the known aggravations among tribes, I expect that governing larger domains will be something akin to the infamous "herding cats." Political stability will likely depend on the vagaries of player intentions, interest, and competence, which together lead me to predict 80% chaos and 20% moderate-to-high stability; the accuracy of this prediction depends on aspects of the game mechanics that I do not currently have (and frankly may never have) sufficient knowledge of.

The behavior of NPCs is critical to the stability, or lack thereof, of society and culture; that behavior is emergent, as are the societies and cultures that depend upon that behavior. I agree that we cannot know now how those aspects will hold up under player-character wackiness, no matter how well designed the base algorithms turn out to be.

I would assume NPCs to be weighted towards stabilizing the status quo with a sprinkling of banditry here and there. To throw out some completely made up numbers I will go with 10% moderate to high stability, 60% controlled chaos and 30% chaos. As politics are an integral part of the game I am running with the assumption that SBS will have a system intrusive enough that even active avoidance of the dance will still lead to the tax collector finding you and that would be a good thing. My ideal for the game would be that even unpopulated by players the simulation would still continue on and generate new content within its own interactions. For it to be able to do that it also needs to be both resistant to player actions and act as an effective limiter on player excess.

Plus it would just be cool to play a game where that fluff NPC Bob the Builder found the vorpal sword in the village well and went off to slay the jabberwock because well someone had to.