Those are government bonds, but it's not far off your idea.
Edit: for clarity, the effective rate paid does vary over time according to interest rates, which usually have some relation to how well the markets are doing.
> interest rates, which usually have some relation to how well the markets are doing.
Uh, maybe? There is a positive correlation of sorts, but it's nothing to write home about. It doesn't seem like this product was designed with any attention to the potential of better-than-fair payouts.
Those are government bonds, but it's not far off your idea.
Edit: for clarity, the effective rate paid does vary over time according to interest rates, which usually have some relation to how well the markets are doing.