If China’s new zone did not include disputed maritime territory, if its requirements for compliance applied only to aircraft heading into Chinese airspace, and if neighbours like Japan and South Korea had been consulted ahead of the announcement, then there would be little or nothing for others to object to. Indeed, it could have been part of a wider strategy of cooperation to reduce maritime security risks in North Asia.
Another important point:
It looks like a pretext for one of two undesirable security outcomes. If foreign aircraft now regularly obey the new Chinese rules, we will see precedents set for the unilateral expansion of Chinese authority over contested maritime territory. Alternately, if foreign aircraft contest or ignore the Chinese zone and a dangerous or deadly incident occurs (such as a collision or a forceful encounter), then China will have prepared the way to absolve itself of legal or moral blame, making it easier to use the incident as a justification to escalate the crisis if China so chooses.
Food for thought. One of China’s long-term goals in the east Asian security arena is to challenge and push back American military supremacy in the region, just as the US military did to European powers in the Caribbean in the 1900s. The South China Sea is China’s Caribbean, and Beijing wants sovereignty over it. Declaring an ADIZ that included disputed territory was a step in that direction, but so far it appears that China’s military is unable or unwilling to protect it with force from Japan and the US.