I just read the top answer to this question, written by u/Steelcan909. In it, he/she says that the Anglo-Saxon migration into Britain was previously seen as an invasion of then-Celtic Britain, but is now seen as a peaceful fusion between the Celts and the Germanic people. To me, that sounds very similar to the claims of a generally peaceful 'transformation' of the Roman Empire into early medieval Europe, rather than the traditional narrative of catastrophic collapse. A third example is the Indo-Aryan migration into India, where the previous concept of a violent displacement and enslaving of the indigenous (possibly Dravidian) inhabitants has been replaced by a model of a peaceful acculturation.
What is the basis behind these reevaluations?
Honestly I would say more the opposite. Starting with the 70s or so there a tendency to replace early, racial models of different "peoples" replacing other "peoples" (eg, the Germans "replaced" the Celts) with a more sophisticated understanding that, as the cliche goes, people are not pots. That is, a change in material culture (say, the way pots are made) does not need to mean a change in population. Or as one of my professors put, someone in Saudi Arabia does not become an American because they drink a can of coke.
Pretty much everyone agrees this was correct and right, nobody wants to turn the clock back on the argument. But recent advances in scientific methods, particularly DNA analysis and tooth enamel analysis, has shown that people moved around a lot more than you might expect, and that near total replacements of the population did happen much more than was thought. To give a notable example, DNA analysis shows that the spread of the Bell Beaker Culture (a major archaeological horizon, and entry point of much of Western Europe into the bronze age) was accompanied by a major population replacement--the modern people of Britain have very little to do genetically with the people of the Neolithic. But here is where it gets interesting, on the continent the spread of Bell Breaker culture was not accompanied by population replacement. So you have the same culture process in some places be the movement of ideas between peoples, and in others the actual movement of peoples.
The broad lesson, I suppose, is that we cannot assume that any two processes of cultural change are alike. And that is for a pretty straightforward question, like "did the spread of this culture happen because of the physical spread of people", how much more difficult would it be for a more complicated question about how much violence a cultural change was occasioned by? I will say that literally all the examples you give are very hotly debated--like I definitely do not know as much about the topic as Steelcan909 but I would be very surprised if they said nobody was on the other side of it.
As far as I am aware nobody has ever tried to tie changing views towards past cultural shifts and migrations to currents within the historians own society. There is a pretty obvious subtext to the "pre-war they thought this but in the 70s they started to think this" though (sometimes not even subtext, as in discussing cultural change in the Roman provinces).
[Here is the paper regarding Bell Beaker genetics] (https://www.nature.com/articles/nature25738).