People who say "There wouldn't be a Russia tomorrow", what do you actually expect countries to do? Launch a hundred nukes at Russia and kill millions of people because of a decision their leadership made? Get in a nuclear fight with Russia and accept dozens of nukes coming back to hit your own country because of something they did to someone else?
Launching nukes would obviously be on Putin's head, any response would have to hit directly on Putin and/or his military. Blowing up random cities wouldn't just be a massive human rights violation, it also would have little impact on Putin himself. What does he care? You got to hit him where it counts, not just randomly carpet bomb the largest land mass on Earth.
Gotta think this shyt through. Not saying there would be no response but it is far more complex than you think. If the USA or anyone else has a special "assassination" button for Putin (targeted strike, deep-cover asset), they probably call in that shyt. Other than that, though, I'm not sure even American leadership know exactly how they would respond depending on how many nukes Russia used, where they used them, what we thought they were going to do next, etc.