A smart manager won't just let their budget be cut, because that won't be good for the company. If they find that marketing approach X doesn't work as well as Y

A smart manager won't just let their budget be cut, because that won't be good for the company. If they find that marketing approach X doesn't work as well as Y or Z, they will simply move funds to do more of Y and Z. By doing so they will increase revenue and profits all the while keeping ad spending at the same level.