The day a nation is able to use a nuke without retaliation is the day the world really goes into a downward spiral into shit. I also think its just not possible. If Russia used a nuke on non-NATO territory there may not be a nuclear retaliation, but certainly a severe and necessarily disproportionate conventional warfare response with the threat of nuclear escalation if Putin uses more nukes. MAD doesnt work if nukes become fair use, not to mention it would immediately spark very serious and probably actionable considerations of a nuclear preemptive strike. In the current state of things, no one is using nukes, so a preemptive strike doesnt make sense unless we had concrete evidence an adversary was preparing to launch their own. But once a nation shows the world they are willing to use nukes against their enemies, even non-NATO ones, the sense of urgency to plan and ultimately carry out a preemptive strike escalates exponentially. I think the moment Putin uses a nuke, nuclear war becomes near inevitable, which is hopefully why Putin hasnt used one despite many years of saber rattling and clearly struggling in this military operation gone wrong.