In financial theory the "risk free rate" is usually proxied as the yield on government bonds. Since the US government is able to print its own currency, it will not ever need to default on its own debt which is denominated in US dollars.
Short of world going dark, you will get your principal and interest back.
Riskless? Not quite. If the total dollars you get back buy less than they used to in terms of goods and services you have lost wealth in real terms.
Investing firm Newfound Research has a mantra:
Risk cannot be destroyed, but only transformed.
This is clear when you consider the trade-off you must always face —the possibility of "failing slow" or "failing fast".
When you are young, you risk "failing slow":
The primary risk of investors with growth mandates (e.g. investors early in their lifecycle) is “failing slow,” which is the failure to grow their capital sufficiently to outpace inflation or meet future liabilities. In this case, our aim should be to diversify as much as possible without overly de-risking the portfolio.
As you approach retirement, you risk "failing fast":
For investors taking withdrawals (e.g. those late in their lifecycle), the primary risk is “failing fast” from large drawdowns. Diversification is likely insufficient on its own and de-risking may be prudent.
Newfound describes risk as "a blob that is spread across future states of the world. When we push down on that blob in one future state, in effect “reducing risk,” it simply displaces to another state."
Entrepreneurs and businesses incur risks that span from minuscule to extremely speculative. This provides investors like you a spectrum of opportunities to partner with them according to your own appetite. An appetite that balances how much return you require in balancing your tolerance for losing fast or losing slow.
Without the possibility of loss, you would not be afforded a chance to earn returns.
Newfound compares investors to the management of insurance companies:
In many ways, we can think of ourselves and our portfolios as insurance companies: we collect premiums for bearing risk...When we buy stocks, we are really trading a certain cashflow today (the price) for a stream of uncertain cash flows in the future. The discount between the price we pay and the net present value of future cash flows is the premium we expect to earn.
The specter of risk and prospect of reward are 2 sides of the same coin
- Newfound Research's No Pain, No Premium (Link)