correct me if i'm wrong but don't you think Qin Shi Huang standardized written AND SPOKEN language? And that that original language just evolved into different languages as the times went on and ppl just did not noticed it?
Remember back then chinese ppl moved in all directions and mass communications were not yet known ...... so it is possible that Qin Shi Huang might have unifed BOTH spoken and written but as times went on that original chinese language that was spoken in the Chin dynasty just evolved into several different languages...... just like how latin evolved.......
besides how could you just unify written and not spoken ? It's like telling everyone to write all theiy words exactly in english but just say it in frnehc or chinese.......
it's not possible in my opinion....

Well, for one thing, according to historical records, it's only been mentioned that Qin Shi Huang unified the Chinese writing system but there was never a mention of Qin Shi Huang unifying spoken Chinese.
Since during the Warring States era, each state had their own version of Hanzi so it's safe to assume that they pronounce them a little different from each other too.
As far as I know, it would be difficult to unify spoken Chinese during the Qin dynasty some 2,000 years ago because 1.) Hanzi is not a phonetic script so there's really no way to know right off the bat how to pronounce a character and 2.) Qin dynasty didn't last too long so while unifying written Chinese would've been feasible, forcing everyone to speak the same dialect would've been tougher. Heck, Mandarin has been the standard language for almost 100 years and it has been popularized by both the ROC and the PRC yet other dialects still exist and some older folks still can't speak Mandarin.
Hanzi is not like Latin letters, Hanzi focus on meaning and not phonetic pronounciation like Latin letters. This is why Japanese, Korean and Vietnamese were able to use Hanzi while not having to adopt spoken Chinese. So your argument of pronouncing words in different languages is irrelevant.
With all that said though, I do believe all Chinese dialects came from the same origin and eventually developed in their own paths. This can be proven through geographical feature of China, historical documentations and simple logic.