One factor complicating a nuclear terrorist attack is the need for the aggressor to maintain command and control over their weapon at all times. Letting a nuke out of sight of a terrorist leadership cell is the supreme act of faith in the attacking cell. The nuke can be turned against them in a leadership struggle; diverted for sale or used to extort vast amounts of money. Fanatics can seize them to use against another faction. Much terrorist violence in the world is faction-on-faction. They can even be intercepted and made to blow up in their own faces. Unless used immediately nukes must be secured in a heavily guarded, hard to conceal place. If used immediately they cannot be stockpiled into a decisive amount. Missile delivery systems were ideal solutions to all the problems of command and control problem. That is why North Korea and Iran sought a missile capability immediately. Anti missiles defenses were therefore an immense discouragement against nuclear terrorism in their own right.
The most likely reason for Russia’s objections to US missile defense is not that it degrades their vast and unstoppable arsenal, which remains effective in any case, but it reduces the effectiveness of sock puppet proxies who threaten the US. Russia is not about to threaten the US directly. But wouldn’t it be convenient if others would? And wouldn’t it be even more convenient if the US could not defend against them.