The point of bringing up the treaty is just to point out that the result of the situation you are describing was so scary that for about 30 years the 2 biggest nuclear powers agreed not to do it. That is all to say that one answer to your question is " US and Russia pretty much saw your scenario resulting in inevitable full scale nuclear war"

US and Russia used to have a treaty against either country developing anti-ballistic missiles. The idea was that if 1 party trusted their ABMs too much, they would no longer care about a counter attack, and that would undermine the MAD doctrine.

https://en.m.wikipedia.org/wiki/Anti-Ballistic_Missile_Treaty