Bonds may have gone down, but they still paid out. Unless people sold at a loss for liquidity or could’ve timed the market and bought low, they were better off than people stashing money in a mattress or most bank accounts.
The simple term for this is Sharpe Ratio https://en.wikipedia.org/wiki/Sharpe_ratio
If you want to get really deep into investment theory, Beta implies basically everything is a good investment in small enough amounts https://en.wikipedia.org/wiki/Beta_(finance)
Though in practice, Beta is mostly irrelevant beyond a relatively simple portfolio https://finance.yahoo.com/news/warren-buffett-myth-more-why-...
Yes, not everything is on the efficiency frontier for risk/reward. Eg cash is convenient to spend, so it can 'get away with' offering a worse risk/reward profile.
Yes, the Sharpe ratio is one of many ways to measure risk adjusted returns.
> If you want to get really deep into investment theory, Beta implies basically everything is a good investment in small enough amounts https://en.wikipedia.org/wiki/Beta_(finance)
Well, that assumes no fees, transaction costs and taxes, I think. And even without transaction costs etc, the theoretically optimal amount can be so low that it's not worth bothering with.