The definition of default is to not be able to make loan payments. It's something the US has never done, and isn't considered likely to do in the forseeable future. The largest risk the US has in this regard is political; politicians occasionally use the routinely-raised statutory debt ceiling as a game of chicken.

> How you think the debt load might shrink without the US defaulting.

It won't, and it doesn't have to. We pay off a trillion on-time, we borrow another. As long as we make payments as required, no one gives a shit.

> And why will the US not just default...

Because then we stop being able to borrow when we want to, like Argentina.

> What do you think the difference is between borrowing a large amount, returning a small amount and claiming it isn't a default?

We borrow a large amount, and return a small amount lots of times, paying down both some principal and a little interest. Again, just like a mortgage.