Is it not an existential risk? To allow their enemies to build up strength on their borders has proven to be an existential risk in the past, although obviously they triumphed in the end, albeit at great cost.

The worry as I have heard it that Russia may not be willing to accept defeat in the Ukraine and will use nukes in the Ukraine, not USA or Western Europe - which is of course bad enough in itself - and that this might escalate into a global nuclear exchange.

Your final point is interesting - I suspect it is a logical artifact. The people who are worried about nuclear war are generally people who want to minimise death; and people who want to minimise death generally prioritise peace; and people who prioritise peace are generally labled as wanting Russia to win, as they would give Russia concessions if it means fewer hundreds-of-thousands of deaths - and anything other than a total victory for Ukraine is said to be a victory for Russia.