I think NATO's position has been pretty clear: they will hold to Article 5, and an attack on a NATO country is an attack on NATO, nuclear weapons threat be damn

I think NATO's position has been pretty clear: they will hold to Article 5, and an attack on a NATO country is an attack on NATO, nuclear weapons threat be damned. The reason we have not committed ground troops to defending Ukraine is because they are not a NATO member. And the reason there's that line in the sand is because NATO policy members are acutely aware of the dilemma that you pose, that if they perpetually back off of treaty obligations because of the threat of nuclear war, there is no treaty and no defense against a nuclear-armed power. I think there's a more uncomfortable "rational" conclusion lurking in your post: nuclear war is inevitable. At some point Putin is going to attack a NATO country (probably one of the Baltics, or Poland) that he believes is rightly part of the sphere of Russian influence, where he believes its NATO membership is illegitimate, and probably also that NATO won't come to its aid. And then NATO will be forced to respond or lose all credibility, and then we've got WW3 and probably nuclear annihilation. This is the problem with MAD in general: humans are fallible, they have inaccurate perceptions of how events will play out, and eventually errors will be made. And when errors destroy humanity, well, that's a problem.