Unless there was a cataclysm that destroyed our entire technological structure, I don't think that a world war could have any significant effect. While it's possible to continue mining cryptocurrencies and continue using mobile phones or personal computers to send or receive them through the internet, I think that everything would continue more or less the same. Regarding the price, which is what many people care more than anything, you can think that a world war would cause an economic depression and a lot of uncertainty, this could make many people think of cryptocurrency as a refuge, it's a possibility, but I would not bet completely for it either.
I dont disagree completely with your point of view, but you will realize that a world war will definitely get Russia involved. And Russia as we all know, is one of the bigwigs in the cryptocurrency society. Any devastating effect of the war on Russian economy will in one way or the other, affect cryptos at large. You are talking about people rather taking cryptos as a refuse, this to me i think will be somehow difficult. Can you imagine yourself running for your precious life with your computer in your hands? To me, when war happens, food and shelter will be the main priority irrespective of whether you deal in cryptos or not. Also think of what happens when servers are destroyed and you lose your internet connection as well. The long and short of what am trying to say is that, when one of the bigwigs in cryptos is affected, it negatively affects cryptos as well.