If the risk is too high, no it is not, almost tautologically so. If nothing is left of humanity but a smoldering crater, then democracy is, and always has been, entirely meaningless. And the survivors (if any) would never forgive us.
Your logic is left over from pre-nuclear times, when it still made sense. As horrifying as it might be be to face this cold, post-nuclear calculus, we must do it to survive as a species. Which is not to say that we should not push back against aggressive dictators — we must, as much as we are able — but conventional warfare between nuclear powers has to forever be off the table.
Rule by a dictatorial kleptocracy renders humanity entirely meaningless. We must always fight against dictatorship and must never let it win. Better dead than yield to dictatorship.
It renders humanity less meaningful for a temporary period of time. Empires and regimes always dissipate eventually. Nuclear annihilation renders humanity meaningless forever. It is an unfathomably selfish preference, especially given that countless humans with meaningful lives have lived under kleptocratic dictatorships. I do not consent to be part of your death wish.
(But I do desperately hope and pray that Ukraine somehow manages to beat Russia back without massive loss of life.)
So "serious" = "high"? Yeah, then of course I agree. The trouble is only determining is the risk high/serious or not.
On the whole, I lean towards Putin not being quite that crazy -- and hey, remember that even the Soviet Union had that ultimate safeguard, officers willing to disobey orders -- so it shouldn't be all that high. But it's hard to say for sure.
Your logic is left over from pre-nuclear times, when it still made sense. As horrifying as it might be be to face this cold, post-nuclear calculus, we must do it to survive as a species. Which is not to say that we should not push back against aggressive dictators — we must, as much as we are able — but conventional warfare between nuclear powers has to forever be off the table.