This is very common. The Romans did not genocide and repopulate everywhere they conquered, for example. A lot of romance language speakers genetically contiguous with Celts and other such things.
And going back further there is evidence of genetic continuity with pre-Indo-European European populations too.
The picture with the Indo-Europeans becomes a little clearer when you distinguish between Y-chromosomal and mitochondrial DNA. The genetic continuity with pre-Indo-Europeans is entirely on the mothers’ side; the Y-chromosome DNA of that population was entirely replaced with that of Indo-Europeans.
True but something like this is hard to accomplish without a centralized state or other institution which encourage assimilation.
Prior to that it’s more likely that the invaders killed and/or subjugated most of the mean and put themselves at the top of local hierarchy meaning that they and their descendants were more likely to have successful offspring.
Based on genetics that seems to have been the case in Europe.
But given that Finland's neighboring languages (Swedish & Russian, the languages of the conquerors) are IE, wouldn't one (plausibly) expect to see an IE language atop a non-IE gene pool ? Instead one sees the opposite. So does this mean a strong link (of some sort or another) between Finnish and Saame (Lapp) ?
And going back further there is evidence of genetic continuity with pre-Indo-European European populations too.