this happened the way it did was because the mangaka didn't handle Giriko and Arachne in a good way.
Giriko for one who waited 800 years for Arachne to have his wait be useless when Arachne just dies off like that. Like Arachne said her self abandoning her body was a mistake and what made her think abandoning her body was the right thing to do... madness alone could not gain victory when the man with the most madness Asura has not even technically won yet just escaped. Arachne was way to strong and smart to have died like that at the hands of the mangaka who made her die via Maka who really is not Strong. Arachne was one to have fought Shinigami and was smart enough to split and survive...
Giriko on top of that one of the original weapon capable of fighting on par with Justin a genius death scythe was conveniently put to sleep during a main battle, and forced to join another big group as a lackey since he had nothing better to do and is made to be dumb...
basically Giriko and Arachne stories were done badly.