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Why aren't they sustainable in the most influential and wealthy country in the world?


Did you notice that I specified "under the current parameters"? If you change the parameters, you change the economics and that changes the math. US defense spending is certainly part of the equation, but other things like virtually unfettered immigration that brings in millions of the world's neediest people create a significant drain on economic resources. There is no single solution to any of these problems, but it seems that part of the solution mix is going to be increased taxes and reduced benefits, coupled with a realignment of priorities. It won't be pretty, but it will be necessary.