There are a number of theories regarding the Neolithic decline and subsequent Beaker replacement, but most of them revolve around a plague and/or famine wiping out the Neolithic peoples (maybe even the predecessor to the black plague), so it wasn't as much a deliberate replacement but more of a "oh look, free real estate" situation.

Here's a couple sources on that:

https://en.wikipedia.org/wiki/Neolithic_decline

https://www.technologynetworks.com/genomics/news/earliest-strain-of-bubonic-plague-bacteria-identified-in-neolithic-site-350328

https://www.nature.com/articles/d41586-018-07673-7

Population replacements just don't happen without some outside force killing a bunch of people beforehand, or a level of genocide that would make Pol Pot blush. If you have any other examples, I would love to hear them. And I don't mean that to be snarky, I legitimately want to know if there are other historical precedents of a non-disease, non-genocidal population replacement so I can wrap my head around this. Because your earlier point was right. For all intents and purposes, it's as if the Celts never lived in England in the first place. How can that be?