When the Fed wants to stimulate spending in normal times, it uses reserves to buy Treasury bills in the federal-funds market, reducing the funds' rate. But as the rate nears zero, Treasury bills become equivalent to cash, and such open-market operations have no more effect than trading a $20 bill for two $10s. There is no effect on the total supply of "quality" assets.
A dead end? Not at all. The Fed can satisfy the demand for quality by using reserves -- or "printing money" -- to buy securities other than Treasury bills.