You might find this site as fascinating as I did;
According to this source, not only is THAAD capable of defending against ICBMs but the AEGIS SM-3 missile is quite a bit more capable than THAAD, and the whole of the US can be defended by just a few missile ships.
I don’t know if this is accurate information, but
Take it for what it is worth.
I don’t know how good THAAD is but I worked on part of it and it is very effective in the testing so far.
At any rate some west coast protection would not be a bad idea.