GMD is only capable of taking out 10-20 missiles at once. Russia and China both have hundreds of missiles, and if the DPRK isn’t there yet they will be soon. GMD is essentially useless at preventing a first strike. It was ostensibly meant for if a rogue state managed to launch a handful of ICBMs at the US, a scenario that really isn’t likely. What it would be useful for, is if the US were to strike first and take out the majority of the enemy’s missiles on the ground. To be clear, I don’t think that is why the US built it, and that isn’t part of the strategic thinking. But that possibility is exactly what Russia and China suspect about GMD. Hence their expansion of new weapons.