The key here is time and pressure, which create different conditions and as you may expect this is a well studied topic today. Let me run you through the decision making process.
The decision makers (National Command Authority) receive the early warning messages. They can either (1) believe that the warning is true and launch their quick responding and vulnerable weapons (~360 RVs for US, ~830 for Russia) or (2) to wait untill there are confirmed reports of nuclear detonations happening on the home soil, ie the point that they are certain that the attack was actually real. This decision has to be made under 5 minutes in most scenarios because you need to transmit the launch orders sufficiently before the enemy attack arrives to launch.
The second option is preferable as under it there is less risk of launch on false warning and this risk is significant as there were several historical cases where there was false warning (ie 1979 NORAD alert) or where a peaceful launch was misinterpreted (1995 Norwegian sounding rocket incident).
However under this second option Russia (this is less of a problem for US) has only ~210 RVs if we use reasonably optimistic assumptions regarding SSBN/TEL alert rates, silo survivability and so on.
The problem here is that US may conceivably build a system to stop said ~210 RVs from arriving to CONUS and hitting targets there.
This leads to leadership requiring to make a choice from two bad options - either risk the launch on false warning and killing millions in both countries over nothing or to drop the threat of adequate response - which undermines the deterrence.
Historically Soviet and then Russian leadership has been in favour of the 2nd option, which lead to development of survivable command and control (so called dead hand system), other systems, formed the response during the 1995 crisis etc.
The deployement of Avanguard in such a way would make this option viable again.
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Grumpy as always.
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