It's only simpler for you to think about. "Missile sponge" is simpler plan to execute and operate. ICBM's are expensive to build and maintain.
Chinese are expected to moderately expand their arsenal, but they don't need significantly more silo-based warheads for credible deterrence, they just need to increase survivability from decapitation attack. Missile sponge is very cost effective way to do that. Their road-mobile and rail-mobile DF-41 ICBM's provide already good deterrence, but putting the new DF-41s into silos just increases the variety and survivability of their ground based deterrence.
>>It's the most clear, and logical explanation standing any military theory test.
Nuclear proliferation is extremely dangerous, for every one in the world including the proliferater, so it's not clear that a country maximally expanding its nuclear arsenal is the most logical move to advance its self-interest.
Chinese are expected to moderately expand their arsenal, but they don't need significantly more silo-based warheads for credible deterrence, they just need to increase survivability from decapitation attack. Missile sponge is very cost effective way to do that. Their road-mobile and rail-mobile DF-41 ICBM's provide already good deterrence, but putting the new DF-41s into silos just increases the variety and survivability of their ground based deterrence.