There is nothing left to collapse. There is only 1 city left on Earth. The Traveler, though awake, is in shambles. Any more "collapse" and there is nothing left. Gaul, a freaking Cabal, was almost able to end it all. Maybe we all deserve to be wiped out.
To be fair he was the “greatest pit fighter in cabal history” who set up the coup with the consul to overthrow calus....with the resources from the remnants of calus’s empire (they made the oxa machine, planet destroying weapons, the leviathan, and overall constant paradisiacal type living circumstances... so they were chonker resources) they attacked... That was “technically” the full might of the now militarized cabal empire. The fact they overtook the traveler was no surprise. Gameplay wise ghaul died from one celestial nighthawk shot i think soooo... lore wise it makes sense but I think they represented it poorly. If it was made like forsaken was i think it probably would have been a little better, or like if were a raid or something... being as it was campaign it makes sense