The Tree of Probabilities is a Mercury strike, the Vex timestream environment that Destiny's Curse of Osiris expansion built its story around. The Epic boss fight format means the boss has been scaled to the boundary of what the build can handle, the encounter designed to find the ceiling. We lost Earth once is the Red War's legacy — the City fell, the Light was taken, the recovery was not guaranteed. The determination encoded in won't lose it again is what the Epic modifier is testing: whether the resolve is real or whether it breaks at a certain power level.