I dunno about replacement theory. That requires genocide. They have no evidence of that. Otherwise, despite invasion, the invaded still remain. That’s been my experience.
“The conclusion that there was almost complete replacement of DNA at this time is pushing the data a bit too far. However, this has certainly triggered a renewed debate about the Beaker. We just need more data.”
This point was backed by Linden. “This apparent replacement is very striking, but it is possible our results are being skewed. In particular, the introduction of cremation at this time could have destroyed bones that would otherwise have provided DNA samples and which could change results. This is certainly not the end of the story.”
You’re entirely correct that the “invaders” are usually absorbed into the gene pool of the invaded. There were reports a few years ago that much of the British genome, after invasions and domination by Romans, Germanic tribes, Danes, and Normans remains remarkably Celtic. But contrast Mexico where the Spanish conquered but left a country largely inhabited by Aztecs and other natives with the United States where disease and war has left just a small remnant of the pre-Colombian population and genetic heritage. If the invaders kill off, by whatever means, the existing population their line can be cut off. Rare maybe, but there are examples.
When you realize how vulnerable isolated societies are to infectious disease, it’s entirely possible that the ‘invaders’ simply brought over some unknown disease. If you look at how far down the native populations of the Americas dropped after Columbus, you realize that it wouldn’t take much if the population were a bit smaller to begin with.