All the recent palaeo-DNA data suggest a horribly massive process of genetic replacement of the local population by the new arrivals. This process is of course very uneven -- e.g., the population of Ireland seems to have mostly shifted to a new IE language -- but in some cases the change was drastic. Moreover, in some parts of Europe this seems to have happened several times, with first agriculturalists replacing local hunter-gatherer populations and then IE people replacing them in turn.
The problem of IE is of course very abstract, while the problem of, e.g., Celts is much more concretely paradoxical (continental and island Celts share the language family but not a lot of archaeology and a dubious amount of genes). However, it is still a more or less commonly accepted fact that at some point in the past PIE peoples spread like wildfire, bringing their dialects, genes, and culture to a very large area, and it is of huge historical interest to know where they started from.
The fact the IE epic and mythological traditions have zero memories of all this, I would say, is interesting but does not prove or disprove anything.
The problem of IE is of course very abstract, while the problem of, e.g., Celts is much more concretely paradoxical (continental and island Celts share the language family but not a lot of archaeology and a dubious amount of genes). However, it is still a more or less commonly accepted fact that at some point in the past PIE peoples spread like wildfire, bringing their dialects, genes, and culture to a very large area, and it is of huge historical interest to know where they started from.
The fact the IE epic and mythological traditions have zero memories of all this, I would say, is interesting but does not prove or disprove anything.