A nuclear exchange would be an extinction level event
A nuclear war- even a fairly limited one- would end human civilization- probably permanently.
The ONLY reason Putin is allowed to get away with these atrocities is that he has nukes, and while the civilized world recognizes the impossibility of nuclear war, we can't be sure that the barbarians in charge over there understand that.
M.A.D. is only a viable (but supremely immoral) strategy when all parties are sane and rational- but if one party is insane, or convincingly pretending to be insane, they win.
We could end Putin's conventional army very quickly, but at the risk of a nuclear war that would kill billions, billions that have no part in this conflict.
So we can't risk it, but I fear we might.