I think if they do that, it only going to force them to make new fight with initial even lower drop rate next time, and ask players to increase drop rate by doing it in reverse-buff mode in order to has little chance of item.

You know that the fact that SE design low drop rate since the beginning is to prevent players from get it fast right? It is their goal that you can not get mirror. So if they give a new method to help, they gonna lower the initial rate in order to keep the same pace.