In case of a world war, your worry applies not only to cryptocurrency but to all aspects of life including traditional finance. Survival will be the top priority over worry about cryptocurrency. Though not all servers will be destroyed, crypto transactions will certainly be disrupted in such a scenario. However, I doubt that the entire network of cryptocurrencies will cease to exist. A post-war reconstruction might take time but human resiliency will find a way to bounce back from such a destruction. Also thinking about alternative communication channels without internet infrastructure. How about satellite nodes that don’t rely on ground servers?