I think the reason it's considered destabilizing is that it promises the possibility that the US could launch a first strike against Russia and then defend itse

I think the reason it's considered destabilizing is that it promises the possibility that the US could launch a first strike against Russia and then defend itself against the inevitable counterattack. So even in a defensive configuration, it has ostensible offensive value. Of course, the keywords here are "promises" and "ostensible". It will never work well enough that a rational person would depend on it. (Note, in particular, that it can never be fully tested, short of actual nuclear war.) But can we assume our leaders will always be rational?