This answer argues about Russia's concerns over missile defense systems due to undermining of Mutual Assured Destruction:
The basis for the modern stalemate is called Mutual Assured Destruction. Under MAD, neither side can start a nuclear war because both sides have sufficient firepower to destroy the other side even in the face of a first strike. I.e. they can each fire in response before the first strike reaches them.
Missile defense systems threaten MAD, because they can (at least theoretically) make it possible to fire a first strike and not be destroyed in turn.
This seems like a valid point, but I am wondering if this cannot be counteracted by also building such systems. It seems unlikely for NATO to give up already deployed systems, so instead of issuing concerns why not build more such systems?
Question: Why doesn't Russia build more missile defense systems instead of complaining about NATO's systems?