That's maybe why the massive IE movements were not so massive after all.
So how do you explain that R1b makes up nearly over 80% of the lineages in places like Ireland or Gascony, and over 50% of the Western European population ? Not massive enough ?
We still can't tell which mtDNA lineages are Indo-European because mtDNA haplogroups were very similar between Eastern and Western Europe, and mtDNA phylogeny is infinitely more intricate than Y-DNA. There are already over a thousand mtDNA subclades and it is growing exponentially !
However it makes sense that the invaders should kill indigenous men, use them as slaves, or marginalise them, while marrying, raping or in any case procreating with native women. Men are not limited like women in the number of children they can have. Having power usually equals having more children. This was all the truer in primitive societies.
So the Indo-Europeans may have been a relatively small ruling class having lots of children with local women, quickly replacing the indigenous male lineages. Or there could have been a massive exodus, with as many of more Indo-European women contributing to the modern genetic pool of Western Europeans. I think that the former scenario makes more sense for three reasons :
1) mtDNA does vary considerably across Western Europe (although we still don't know what each subclade correspond to), meaning that the pre-existing diversity survived the Indo-European invasions (as opposed to Y-DNA).
2) People do look different in various parts of Western Europe despite strong similarities in Y-DNA haplogroups. This could be due to recent natural selection to adapt to different climates (lighter pigmentation in the north), but 4000 years is a too short time.
3) Invading armies have more men than women, and so must find women in the local population. Wars are waged by men, and the losers suffer heavier casualties, leaving more women available to the winners.