According to proponents of the "big rock theory," the Bank will make large losses when it raises the interest rate on excess reserves because of the low interest rates on JGBs purchased by the Bank in the past. It is of course possible that the Bank may register losses because it will receive low interest rates while paying high interest rates.
Proponents of the "big rock theory" argue that if the Bank does indeed make losses, confidence in the currency would be undermined, there would be hyperinflation, the yen would collapse, and interest rates would surge. However, please think about it carefully. Essentially, does anyone use money paying attention to whether the central bank makes a profit or a loss?