I have a little theory as to what my happen with this arc. In ch. 107, Frieren mentions that after analyzing the Goddess' Monument, it "...feels like a week's worth of memories fade away." It's entirely possible that they may have killed the great demons Todd, Rivale, and Grausam here. They just may have forgotten it happened. We know that Solitar is alive, so she has to live. Once the battle is settled, or even during the fight, everyone's memory becomes wiped due to the interference of the Goddess which may explain why no one is aware of this. Solitar during the Golden City of Weise arc did not show any knowledge of previously knowing who Frieren is through a direct encounter, so I think that's fair enough evidence of this happening.
Also, in ch. 89, we get the Omniscient Schlacht hinting at a "contingency plan for defeat" in preparation for his fight with the Hero of the South that involves the, "sake of the demons a thousand years later." Interestingly enough, Solitar also mentions in ch. 103 that the Great Demon King's desire for coexistence with humans is dangerous and that, "Just a little longer and the demons would have been exterminated by mankind and gone extinct." I think with the reintroduction of these four great demons, we may gather some more information regarding this contingency plan that Schlacht hinted at. The Miraculous Grausam may be the one to spill some of the details as he seemed privy to Schlacht's plan. Although, perhaps not due to Schalcht's clairvoyance, but maybe the Goddess' magic could have interfered with his precognition. Regardless, the demons are probably on the backfoot in the current age, but they may have some kind of trump card waiting for our heroes at Ende. Personally, I do wonder if the demons plan on reviving their fallen king.
I also happen to believe that this entire arc could be interference from the Goddess herself as a way to combat the demons. The Goddess may want Frieren to become privy to their contingency plan and sabotage it in the present day. After all, it would be really unfortunate if everyone, including Frieren, were to forget about these events. I'm so looking forward to the conclusion of this arc though! You heard it here first! (Also, Todd is pretty cute ngl. She's defintely up there alongside Solitar and Aura for my favorite female demon designs)
EDIT - As some have pointed out, it looks like the Bloodstained God of War, Revale, may still be alive and active! Revale bears a striking resemblance to the demon that ruined Stark's village in ch. 26. I'm looking forward to the anime as it may provide further evidence of this belief!
