Even in a WWIII scenario, there would be some form of economy. The composition of the GDP would drastically change. But there would still be demand for food and clothing and of course military goods etc. All these require capital for which investors will collect part of the profits.

So I think a diversified portfolio would still be valid. Even if the stock markets would be closed for years, I could sleep well as some of my companies will go bankrupt and others will have much higher profits.