...where exactly PLAAF need to move their MBTs with such a speed? that comes with a [high] price
Western China, where main rail and road transport arteries represent weak choke points.
Essentially There is only 1 mainline going into Xinjiang from Lanzhou. one well placed ordinance can knock it out.
Right now the heavy armour reserve units (parts of 21 and 47th Group) are all in Shaanxi, Gansu, Ningxia. Xinjiang MR has comparablly smaller force of MechInf. holding as regional reserve and sitting on top of a big logistic pile. Border choke points are held by Mountain and Infantry forces. In the event of an armed conflict 21 and 47th will move their brigade sized units into Xingjiang via railroads and roads... not an exactly comforting thought consider the long road to battle area. also notice 21 and 47 has mostly all brigade sized units...
remember it is not aimed at the central asia stans. Afghanistan and India is the main concern.
PLA is not the multimillion man army anymore. and can't afford to forward place large units.
They have manning level below a million. and actual ground combat units are in around half a million... their ground doctrine relies on timely concentrate mobile armored reserve into theater to deal with any situation.