The article you linked, doesn't say we spent $6 trillion in Iraq, it mentions a report saying the costs could be 6 trillion over the next 40 years. The infrastructure report wants 4 trillion by 2020. Very different time periods.
Money owed is money owed even if interest only becomes 2 trillion we are still at 4 trillion. It will probably end up as more than 6 trillion as these figures are usually off on the low side. That is money that is on the books that could have been used for infrastructure and is harder to get now.