Buddy first and foremost I will have to comend you for such a great thread, I must say I love this, realistically billionaires networth may decline due to the bad state of his or her country economy though it all depends on his versatility in investment because most billionaires are very smart to the extent that they know what the masses needs so before they run out of business or go into any form of decline, they will findout what the majority needs and massively go into the production in other to remain relevant to keep growing their wealth, though I want you to understand even though billonares are broke it will not be to the extent of not being able to afford somethings, it will only affects somethings but  their smartness will make them to bounce back although the bad economy of a country affects a billionaire that only has investment in the country that's been affect, that's why as a billionaire you have to be versatile, your investment shouldn't be centered in one place, it should be in different things and different countries in other to avoid being affected by such an ugly situation.