“Investment income” is one thing, Old Pro, but that differs markedly from the risk-free returns that Treasuries afford the saver, and those that cannot afford even short term volatility on the principle.
Consult your financial advisor for a more detailed explanation.
Treasuries principal varies and would decline in a rising interest rate environment thus providing liquidity at a cost of principal loss. The interest might be risk free, not principal unless the holder waits until maturity. For liquidity the often used strategy employed would be a bond laddering approach.