Well, were are the Mohican or Pequot Indians? What about the Etruscans? The Lost Tribes of Israel?
People come and go.
DNA tests show that up to 5% of European and Asian peoples’ genes are Neanderthal. So they got absorbed. Maybe made slaves or conquered.
Too many bones and other artifacts have been found to say the Neanderthals didn’t exist. And their genomes have been completely sequenced.
What particularly happened to the last remaining full-blooded Neanderthals we will never know.
There are only one or two complete skeletons. There are only about 200 partial skeletons.
One competing theory is that these were extremely old people which would be another reason for sharing DNA.