Where do you draw the line? If you are happy to give up Ukraine to avoid a nuclear war, where do you stop? Can he take all of Eastern Europe? What about the whole of Europe? Everywhere except your country?
Putin is a bully, and you stand up to bullies.
Besides, he might have the most nukes, but given the maintenance costs for 5,000+ of them and the corruption in Russia, most of them probably won’t work.