We already have a proven ability to shoot down incoming ballistic missiles so the DF-21D isn't so much a game changer as much as just another threat and considering how much a ASBM will cost compared to a ASM I don't think they will be able to mount a saturation attack with these yet. Also these are fired from semi fixed sites vulnerable to destruction.
Also the PLAAF's Badger bombers have the possibility of greater range with IFR than the current DF-21D.
The real effect of the DF-21 missiles we will see in the coming decade(s) is the militarization of space. The DF-21 is also the basis for China's ASAT missile, so putting up birds that can shoot down DF-21Ds and KT series missiles will quickly become a priority. If you can shoot down enemy birds in the air/space than very quickly you can start to shoot at targets on the ground. Combat Aircraft provide the air cover for our fleets and the striking force of them, very soon we may seen combat spacecraft providing space cover for our fleets too... then maybe as a striking force as well.
Remember two words "Casabla-Howitzer"
