None of the current ICBM platforms were designed for missile defense. Missile defense simply did not exist at the time.
Sentinel is busting its budget because it’s renovating and rebuilding all of the ground segments: all of those decrepit silos and computer systems. It’s still money well spent in my opinion.
Missile guidance is not a computationally hard problem, and it hasn’t changed much since the 50s. Terminal missile defense is a fantastically hard problem, and wasn’t mastered until the last decade or two. And the current generation missile defense capabilities still haven’t all been demonstrated in combat.
Having said that, I would generally expect NATO’s missiles to work as advertised in a hot war. And I would plan for Russia’s missiles to be somewhat less effective than they advertise, but still a credible threat.