@spendius,
Yields are numbers along a single risk dimension, but there are at least three other relevant dimensions to risk of default:
- What currency is the debt denominated in? Japanese debt is enormous as a ratio to GDP but it's almost entirely in yen, and they can print those as needed.
- Who holds the debt? If the holders are primarily domestic households and financial companies - again Japan - they have domestic regulators and taxes offsetting low yields. The "sterling crises" in the past century came from foreign holders wanting to be paid in currencies other than sterling.
- Finally, how big is the total debt in comparison to assets of the borrower that can easily be monetized? That's where the crucial distinction between
solvency (which you mention) and
liquidity comes in - but in a crisis, such as we're in now, that distinction is lost.