Russia's chief concern is that the ABMs, combined with theater SAM defense, is upsetting their favored battle-plan of decapitating NATO's command and control, preliminary to a limited goal incursion.  This would give the Russians time to destroy NATO's FEBA nukes, and hopefully paralyze their response, as only full-range ICBMs, and SLBMs, would be left ... an escalation which would result in MAD.  The ABMs add a few vital moments for NATO command to fully assess the threat, and to implement "WarPlanEagleGuardian2", or whatever they call it now.  Russia has already demonstrated some skill in "limited goals", and it would be almost impossible for NATO command to justify an escalation from a limited Russian "incursion", to all out Nuclear holocaust.