The issue you're talking about is real.  Bond specialists refer to it as "reinvestment risk":  the risk that, when you go to reinvest the interest payments, you end up having to invest at a lower rate.  (To that, of course, you have to add the "lazy investor risk" and "spendthrift investor risk" that you'll either not get around to investing the money at all, or worse yet, just spend it.  The bond specialists would never do such a thing, so they don't have a name for it.)

If interest rates are higher when you receive your interest payments, you're golden:  reinvest the money at the higher rate.  (Of course, you still have to actually do it.)  Similarly, if your plan is to spend the money (because you're already retired, let's say, and the interest is what you're planning to live on), then getting the interest paid out is just what you want.

If you definitely want to be compounding, and you're concerned that future rates might be lower, there's another option called a zero-coupon bond.  It doesn't pay semi-annual interest payments.  Instead, you buy it at a big discount to face value and then it pays off at face value at maturity.  The result is guaranteed compounding.  I'm not sure you can buy those directly in a Treasury Direct account, although you can definitely get them through a broker.