What they have now is never going to something a anti-missile system can cope with, and they know it. I'm sure there is some geopolitical reason for this, but I don't know what it is.==
Reason is that those huge warheads and huge missiles will retire soon. Then Russia will dismantle them and won't to rebuild them anymore.
With this new warhead Russuia may go to decrease number of warheads further more despite of American anti-misslie shields.
There are NO American anti missile shields. None!