According to international law, one of the most common principle for establishing sovereignty is by "effective occupation and control." These are the main reasons provided by Vietnam, the Philippines, Malaysia, and Brunei to claim their sovereignty over Nansha Islands. This means that in the event of conflicting claims by several claimants, the sovereignty is given to the one who is actually occupying the land. Therefore, if China did not forcibly take back S. China Sea then it will ultimately lose its sovereignty, at least in the eye of the international law. Therefore, there is no time to waste. China must take back and occupy and control all islands in the S. China Sea.
I think the Chinese government will enhance its support from the Chinese people by forcibly take back S. China Sea. It will also decrease the determination of Taiwan to use force to separate. The US and Japan will not intervene for many reasons. First, this is not their fight. Second, they are in deep economic trouble. Third, China can damage their military in they choose to fight. Fourth, they need the cheap imports from China. Therefore, for these and many other reasons it is a good time for China to use force to take back its sovereign territories.
The international community are divided into 3 parts. The enemies of China will of course condemn China if it use forces to evict Vietnam and others. The friends of China will cheer because they see they have a strong ally and protector in China. They majority of the world will not care or will approve and understand that China is only protecting its own sovereignty. Therefore, China will only enhance its own best interest by using force now to take back its sovereign territories in the S. China Sea. On the other hand if China keep waiting, then it will lose respect in the eye of the world and also lose its sovereign territories in the S. China Sea. And lastly it will also lose respect in the eye of Taiwan people and make them less willing to unify with the motherland.