It would be quite interesting if Tian Cheng actually lost his whole raison d'être just like that. I don't like depression arcs, though. If Xuan Xuan and the crimson demon both die, how could it be anything but a depression arc? If Xuan Xuan dies and a weakened crimson demon flees, it would still be a depression arc, just with vengeance. Considering Luo brat's dad, I reckon it could be possible Xuan Xuan and the crimson demon end up fused together, and Tian Cheng needs to figure out how to split them.
Since this is a cultivation story, it would actually be strange of the male main character was able to stay together with his love interest for more than a brief moment. Usually they are separated by circumstances and the bulk of the work is for the MMC to somehow find a way to be reunited. Just like this series was until this very arc. I could imagine another separation, lasting till the very end of the whole series.