Ethnicity and language have nothing to do one with another ?
Ethnicity is a mix of genetic aspects and culture and SELFAPPRECIATION OF IDENTITY -
a human group, to keep some consistance and to pass some centuries before to die, needs some stability ; from this stability and warrant of this stability is language – genetic aspect is also consequence of this stability but not an evident cause of it – genetic aspect found importance recently in a post-colonial context with phenotypes very different and a population density very bigger than in ancient times.
languages COLLECTIVE shifts need tight contacts and exchanges OR a very strong network of administrative organisation, but this last aspect was not already the case among the most of ancient ethnic groups -
- as a result of this (contacts and exchanges, balanced or not), members of the new mixed group with new language are pushed to feel themselves as of the same group as the ones which possessed this language before mixing or imposing this languages along with other things. They adopt the ethnicity of the dominant. Vanquished Gauls feeled themselves Romans some generations after the defeat. Some exceptions exist, like the Franks/Français question, but even here, we see a final fusion of ethnic sentiment, language and in a big part, genetics. So an « elastic » link between these notions, and not a « nothing common ».
So, Balto-Slavic stage is surely not only a linguistic abstract reconstruction only but also an ancient stage of a pop with flesh, blood and bones. After, things evolved, of course...