With Governments and debt because Governments do not have a 80 year life expectancy the total dollars of debt is meaningless. What matters is the debt to GDP ratio, in other words its perfectly fine having deficits that are 3 to 4% of your GPD just as long as the growth of the economy plus inflation is higher than the deficit.
The US economy is almost 18 trillion dollars per year, thus a 500 billion dollar yearly deficit means the economy has to grow 2.77% or the economy has to have 2.77% inflation, or a combination of those two numbers.
Now this is not saying money should be spent in an unwise manner in goverments. We need accountability in government to make sure our tax dollars are being used in the most productive way but deficits are not necessarily bad as long as they are small.
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But trying to re-negotiate the principle of an already accrued debt or saying you are going to not make a payment for an already on the books debt is completely and utterly destructive for an economy.
It causes the people in the banking system to suddenly be underwater with their books when you have a dollar whose value overnight is worthless because the whims of a single man or congress. It would make the lehman situation of 2008 look like a jack russell terrier size problem vs a great dane size problem with the difference of proportions if the US suddenly defaulted on its treasury debt.
It is a huge deal and the fact that Donald Trump does not understand this makes him extremely dangerous. You need to understand the scale of the situation what a person does in their individual life is different than an economy of 300 million people which controls 1/4th of the world gdp and $200 + trillion dollars of assets. A country can't do a bankruptcy in the same way that an individual can.