I guess another way of asking this is: are cultures with written languages more likely to exhibit diglossia?
I'm listening to John McWhorter's course "The Story of Human" language on audible and he's going over the concept of diglossia. I can see how in modern societies how the 'standard' language of government, media and the courts would engender a situation where regional, social and ethnic local dialects flourish. The languages "on the street" would be living, dynamic phenomena while the standard would be relatively slow moving due to historical documents, media...etc.
So, do we know if diglossia was a feature of pre literate cultures? Did the speakers of PIE, for instance, have two different ways of speaking? Is diglossia endemic to all cultures?