Depends which window you look out of.
You could just as easily argue the complete opposite.
Persian was dominant in Central Asia during the Sassanid and later Samanid eras, developed a literary tradition and was the language of pollitics.
Then this Persian dominance was slowly eroded away untill Turkic developed into a literary language powerfull enough to be used in all spheres of governance replacing Persian in Turkistan especially across the masses.
So its a matter of perspective.
In terms of legacy, today Turkic languages are more widely spoken and Turks managed to keep their mother tongue and not assimilate in newly conquered terretories which few nations have managed to do.
Edited by Bulldog - 30-Dec-2007 at 00:06