"Cash" is also not risk free. Probably higher risk than US, Japanese, German, or British bonds. In the current fiat era a country is more likely to print money than default on its bonds. Countries that have recently gone bankrupt or gave bond holder haircut, rarely can borrow in their own currency anywhere near the rates on US dollar denominated loans.

Printing money to pay a debt is defaulting in any sense that would be understood by a layman. The loan has technically been paid, but in a way that voided the point of requesting payment (transfer of value).

Again, the fact that it's not considered that way says more about the finance industry than it does about ordinary terms like cash.

Credibility claimer: I built risk systems for 3 Tier-1 investment banks in a previous life.

I'm not a fixed-income (read: bonds and other interest-rate derived financial instruments) person, but I think the erosion of 'value' via inflation is missing the point.

Sovereign debt is the least risky form of debt there is, period. Everything else that's denominated in the same currency has more risk, not least due to the fact that the pricing of every financial instrument takes the risk of the sovereign debt into account aka the risk-free-rate.

Yes, the absolute amount of risk of a sovereign debt is different depending on who controls a particular fiat currency - but in relative terms, any financial model will start with that baseline. Obviously, cash in that same currency bears some risk due to inflation, which is why people talk about 'real interest rates' that take that into account.

If you lend someone money, you take risk, and you get interest as compensation. That risk includes your predictions of inflation, and the interest rate you are prepared to accept should include that too. If you don't like it, don't lend the money.

Arguing about a 'transfer of value' implies that you have another way of representing value that is better than the currency itself. I can't think of one, but I'm open to suggestions.

Currency is a fine way to represent value, but a currency that's being inflated is certainly a worse way than a currency that isn't. If a government plans to inflate its way out of debt, the markets respond to that by trying to charge higher interest, but ultimately governments (outside the eurozone) control their own currencies and their own inflation statistics, so the markets are always playing catchup.

A truly risk free currency would be a currency that couldn't be inflated by a government to escape from its debts, like a cryptocurrency (if they actually worked).

For most of history there was gold and/or silver. Fiat currencies have always been short lived. Our current fiat phase started in the 1970's so maybe it can survive a little longer?