Whatever happens, assuming modern medicine takes a fairly similar rate of progression there will be a population boom among the Russian central Asians. A long time ago in 1978 or 1979 a science fiction writer (I think it may have been Norman Spinrad but it is a long time since I read it) looked at the official published demographic data of the then USSR and concluded from the demographic trends that, if the Russians didn't cut the 'stans loose, they would become an ethnic minority in the USSR by 2020. So it is probable that changing demographics would have ultimately dictated some form of independence whether from the Czars, Russian Republic or USSR.