The situation only holds when the market price of mammoth ivory is roughly the same as that for elephant ivory. If mammoth ivory was much cheaper than elephant

The situation only holds when the market price of mammoth ivory is roughly the same as that for elephant ivory. If mammoth ivory was much cheaper than elephant ivory, then killing elephants wouldn't be economical. Unfortunately, mammoth ivory is an extractive process, like mining, so it is unclear if the price could be brought down and kept there for an extended period. Still, it is more than likely that mammoth ivory has had a protective effect on elephants; although, I know of no studies examining the issue.