When a company fails the bond holders get paid first. Bonds are also the rich man's way of getting paid by governments. Lending money to finance war has always been a nice way to trap governments.
Unless the government, which has the power to break the agreement, breaks the agreement.
The history of finance is well-woven with the thread of sovereign defaults.
I'm just thinking that by the time a state defaults on bonds the "rich man" may be facing a bundle of problems.
How much would diversification help in those circumstances?
For a while during the 07-09 financial crisis, Coca-Cola bonds were priced by the market as less risky than Treasury bonds.
Large multinational corporations are probably safer than the riskiest countries if that's what you're asking.