We can probably shoot down most missiles from all the countries that have nukes. But there’s no way we can shoot down thousands of Russian missiles. Although they’ll need to replace their warheads soon since their stock is aging rapidly.
Expiration of the potency of nuclear weapons in current possession, is also a concern among most nuclear powers, including the United States.
The current assurance rests on the accuracy of computer predictions. That, in itself, is telling.
Let's talk Russia into using them on China: which is already trying to position itself to take over Siberia by mass immigration.
Cheers!