There is no need for tax. In a system where the government can create money directly, government projects which create "civilization" can be funded by simply creating the extra cash. Doing so devalues the existing cash (because now it's less rare) so this cash creation needs to happen in moderation, but it does work.
And this is how things work now. Tax does not fund the government. If it did, we wouldn't be taking out loans and racking up debt. As this situation proves, tax provides a very small amount of the government funding.
And that's fine. I like the government being able to create money. I like roads and schools and national parks and NASA. In fact, creating money is the perfect tax- the dollar decreases in value and therefore those with a lot of dollars lose more than those with only a few. And there's no administration to be done to facilitate it.
What's not fine is the only mechanism the government has to create money is to borrow it from a bank. People like to rant about which banks it borrows from (the fed, china, wherever) but that's short sighted.
The problem is the fact we end up paying interest on ever dollar the government creates.
And that's what isn't sustainable. Paying tax to pay interest is unacceptable.
The idea behind defaulting is to get away from this ridiculous idea that the government of this nation has to borrow money to create money. Going about it this way would have been horrible, but seriously considering breaking the tie is worth real consideration.