Seeing language from the DNA

[Cejo]

Then, how can we tell, when a dialectal leveling or language shift has occurred? "We cannot see that from the archaeological or genetic data, but only linguistic evidence can tell it."

Yes and again until there are no records! ...

Aren't languages themselves suitable as records of their development? Isn't that partly (maybe even largely) what linguistics is all about? 

Is that the crux of this long-running disagreement?