Before diving into this theory, I want to emphasize the title. This contains spoilers for Volume 8 Episode 5 of RWBY. If you have not watched this episode, this will spoil the big reveal at the end. With that out of the way, let’s get on with the theory.
Much like my Atlas falling theory, I believe that Mantle could fall. At the end of Episode 5 – Amity, it is revealed that a river of Grimm is heading straight for Mantle. True that the Happy Huntresses, Yang, Ren, and Jaune are all very skilled in fighting Grimm, however they will be dealing with a constant flow of Grimm.
I only bring this up because I can only think about one other instance of a constant flow of Grimm, the Fall of Beacon. In Volume 7, Penny said that this will “Be just like Beacon!” Right now we have had: a disrupted broadcast to Remnant (due to Watts’ Black Queen Virus), a headmaster protecting something vital, a group of Huntsmen and Huntresses who the headmaster trusts the most, a Maiden willing to protect to the best of their abilities, and now a way for Grimm to constantly attack citizens and keep coming. Just like the Fall of Beacon, Salem is going to win by overwhelming the Kingdom of Atlas, just like how she overwhelmed Beacon.