English has become a common language only over the last 500 years ~ before that, it wasn't even used in civilized society in its own country, and a thousand years ago, it was actually a totally different language more akin to Dutch and Danish than to what it has become.
The Period of Warring States was more than 2,000 years ago. That's plenty of time for new variations on an existing language to rise up in China. In fact, it's plenty of time for Latin to give rise to French, Protuguese, Gallo, Italian, Rumanian, Spanish and several other languages.
There are thousands of dialects in China and the fact you refuse to look at it, shows just how narrow minded and simple-minded you are. Writteln language is just one, but spoken language there are thousands.
The fact is, even some part of the Chinese written language can be traced back to the 7 warring states. Tell me, if it was just wiped out completely by the Qin, how is it possible that the language is still there, both spoken and written?
Face it, Qin didn't do the stuff you are claiming he did. he did not wipe out everybody else that he conquered.