[yorwba]

France has had hundreds of years to try and eliminate regional languages, and yet there still are people speaking many of those languages, even if they only form a small minority. Language death is a very slow process.

I think you're also missing that the Chinese government's language policy is not driven by a single aim. Making the populace predictable and controllable are reasons for the central government to prefer a single language and culture, but trade with Mongolia and tourism where visitors get to experience living in traditional Mongolian yurts, listening to traditional Mongolian songs and riding horses in the traditional Mongolian way are strong economic incentives for the government of Inner Mongolia to retain the Mongolian language and culture.