I imagine they are working on the premise that if the USA cannot defend against these weapons then they would not need to build an arsenal of planes or tanks or take up large defensive positions. If they can develop anti-missile technology capable of stopping US or NATO (or anyone’s) missiles while we cannot stop their missiles, they have checkmate on the board. We will always need to be capable of either knocking their missiles out, or having non-interceptable missiles of our own... preferably both. We need a MAD stalemate at the very least.
Sure but it has to be economically practical.
This is economic compared to the target but not to the counter measure.