I'm not sure I'm getting your point? Throughout history, invaders, whether mass migrations or local tribe vs tribe tend to assimilate over time. They introduce new language, technology and so forth, but over time they become the people they invaded. Above, you mentioned a 300 year hiatus in construction of henges. Now, that could be because warfare interrupted that activity, or it could be that the invaders eventually assimilated and adopted the culture and resumed the activity. There are plenty of exceptions to that - if one group is far ahead of the other in technology (Europeans vs Native Americans) well, the more primitive culture is smothered. But, when cultures are at similar stages of development, they tend to assimilate.
In the case of many of England's invaders - that's what they've been, invaders...rather than settlers. They've arrived to take over the country as a going concern, and slot themselves in as a new ruling class or group, often by enslaving a goodly percentage of the "natives" (whoever they may be at THAT time) or having them inserted into a new economic and social role... The Romans invaded a Britain known already across the Roman world for its prosperity, which already exprted a lot to Rome; slaves, hunting dogs, and British gold among other things. They established colonna, and parcelled land they took out to veterans etc....but otherwise just inserted themselves in as a "ruling order" - not unlike the British Raj in India taking over the same role as the last of Muhgals. The Normans invaded...and immediately converted England into a feudal fiefdom of the Duchy of Normandy, and "reorganised" the country economically (see the Domesday Book) to fit it into the economic rules of feudalism as it sat then. There was otherwise not much actual displacement of the Anglo-Saxons - but they had been "adminstratively" assimilated - squashed into the Normans' way of reckoning property and holdings. The Saxon invasion was more of a pure invasion; yes, eventually there was considerable assimilation, and in parts of the country it happened with minimal fuss...but in other parts there was tooth-and-nail warfare for several centuries... But this period of Neolithic violence seems to have been pretty vicious, and remarkably prolonged, and I don't think it ended in assimilation - given that the one evident target of the campaign, the causewayed enclosures...disappear out of the archeological record in favour of other settlement patterns Farming continued and became universal, and the change in religious practices became pretty universal...but so far the best that can be said is that these changes had just STARTED happening then, there's not proof...yet...that the causewayed enclosure dwellers were responsible for those changes. Chronologically, that may just be coincidence, there isn't an archeological "smoking gun" on that yet. it could for example have been the other way round - and it was the proponents of the new religious ways that were the attackers! Which would also pretty well explain why the monument building stopped while they went to war... EDIT...just a minor one - these were still socially primitive societies, though organisation....or at least organised effort....on a multi-community basis was immediately round the corner for them The idea of limited war may have escaped them, it might just have been a war of extermination because they didn't conceive of any other way of war at that point...!
Thinking more on it - it's like the difference in emphasis and degree between the Spanish and Portuguese arriving in the Americas, and the Western Europeans; the first party slotted themselves in as a ruling class (a bit like the French LATER slotting themselves into Mexican society in the 19th century)...and ruled/exploited the Amerindian natives...who's lives otherwise remained quite similar to what had gone before... ...but the Western Europeans to the north generally preferred to remove them from their lands and take the land for themselves over the next centuries. Displacement/eradication rather than assimilation.
Actually, the average Spaniard (not the Hidalgo's) did assimilate in Mexico. Most Mexicans are Mestizo (which used to be an insulting term, but is not today) a mix of Spanish and Indian blood. In rural Mexico even today, they still farm and live in Indian fashion. The diet is maize and beans and peppers prepared in much the same way it was 500 years ago. The Europeans brought different livestock, so they raise chickens instead of turkeys and eat beef and pork instead of venison and javelina. They speak Spanish, but other than that and those things changed through technology (TV), the culture is still largely intact.
Well, it was the Hidalgos I was thinking of, and how - not onlike the Normans - they pretty successfully mapped across their European social system onto the New World... And while the "average Spaniard" might have ended up as the Spanish American equivalent of "po'white trash" (for want of a better term, but it does firmly orient them in time and social place)...that level of assimiliation importantly didn't upset the Hidalgos' imposed " layer" in Spanish American society.... ...until it eventually did with a bang, and the whole swathe of 19th century popular movements and revolutions across the South and Central America.