The best estimated guess for a bond ETF’s returns going forward are it’s SEC yield, not past returns. This is because efficient market hypothesis postulates that the aggregate of bond buyers and sellers has reached equilibrium at its current interest rate. So it’s our best guess at the average of interest rate increases and decreases and their likelihood of happening. Given that, we assume the return going forward is what it’s currently yielding, and in VGLT’s case, it’s 2.15%. When cash is yielding 0.5% max, these can be a useful way of further protecting against equity downturns (assuming VGLT increases in downturns, negatively correlated to equities, true the past 30 years, mixed truth over 70 years), while still providing much better yield than cash.