First of all, I must say that I realise that this is not exactly a linguistics question so much as it is an anthropological, sociological, or historical question, but I suspect this might be the best site for asking this question.
What I'm really puzzled about is how PIE retained its vocabulary to near-perfection amid the Indo-European expansion, even though the various IE divisions had encountered dozens upon dozens of independent hunter-gatherer (HG) tribes, all of whom likely spoke their own languages, and Early European Farmers (EEF), who also spoke their own languages.
Genetic studies show that the modern European genotype consists of a mixture of Yamnaya, EEF, and HG ancestry; the proportions vary from region to region, but, on average across Indo-European populations, less than half of the ancestry comes from Yamnaya culture nomads.
Therefore, Yamnaya people must have mixed very well with the local populations. Of course, that means they must also have spoken the same language. However, if that is so, here is my question:
How could the local populations and Indo-Europeans (the Yamna) possibly not have formed a creole language?
Of course, neither an economy nor institutions existed, so the local populations could not be forced to learn PIE by the economy, and nor, of course, could the language be deliberately taught in institutions. By what mechanism was this language shift hence driven?