We should take this with a grain of salt. All of this will happen no matter what the United States does about defenses. Nor has China made any offer to refrain from its offensive buildup if the U.S. refrains from defenses. As for becoming an even more ardent proliferator of offensive missile technology, China would hardly enhance its reputation as a responsible international partner if it did so - not to mention that its bargaining position in this regard suffers from its less than complete compliance with earlier assurances given the Clinton Administration. What is really at stake here is not hardware but geopolitics.
China sees the U.S. network of alliances and commitments in the region as inconvenient.
What it accepted and even welcomed in the Soviet era it now sees as incompatible with its own vision of the region's future. This is an important shift in Chinese policy. The U.S. cannot yield to these pressures without paying a huge price. If the technology exists to defend against these missiles and the U.S. flinches from providing it to allies and friends on the front line, it will be a severe blow to the confidence of those who have relied on us for 50 years. Our alliance relations might not survive it.