The last claim that Russia has to being a superpower is its nuclear arsenal. The first claim that China has for having become a superpower is its nuclear arsenal. If a working US missile defense system is deployed, both China and Russia face a United States freed from the limiting factor of nuclear weapons. Neither Russia nor China is certain that the United States can pull this off. Their problem is that they have seen too many impossible technologies become real after the United States decided to do it.
Nuclear parity is too important to Moscow and Beijing to risk being wrong.
Moreover, if they are wrong, the resources required by each to build similar systems would cripple their other defense spending - if the effort is not simply beyond their economic or technical capabilities. There are too many chips on the table for them to gamble. Therefore, they cannot accept the complacent view of US critics who argue that ballistic missile defense is impossible. They have to act. For both Russians and Chinese, there is a diplomatic opening provided by the missile defense issue.