In the United States, defaulting on Sovereign Debt would basically guarantee the death of social security. If you're in the USA, by far the best way you can insulate yourself against sovereign debt is by:

1) organizing your life's expenses so that you can retire (or at least continue to live) with $0.00 from social security.

2) Ensuring that you don't rely on any federal pension (including military retirement).

Of course, that isn't possible for most people. So in the USA, the answer to "who loses?" is "old people".

3) Ensuring you will have be able to afford healthcare in old age without medicare.