There is ONE hypothesis that is very important in your logic and which may not be true: that Putin WILL invade others “once he is done with Ukraine”. He is overstreched both militarily and financially. And he is FAR from having at least enough power over Ukraine to keep it (for now). So he will be even more over extented and financially drained as weeks go by. So you are suggesting to expose your nation (I am Canadian) to nuclear threat based on MAYBE Putin will continue. Although your reasoning is good, i think the leaders of NATO have chosen to see IF Putin attacks a NATO member, at which point that will be “the red line” and they WILL stop Putin at that point, even in the face of nuclear threat. But their calculation now is that maybe Ukraine will be such hell for Russia that they will simply not be able or willing to go to other countries… so their calculation is simple: why risk nuclear threat when it may not even be necessary? IF (a big big if) Russia attacks a NATO country (Baltics or Poland, etc), THEN things will take a WHOLE different turn.