Ok maybe not kill entirely. What if Denji eats half of Death? Or we get a nerfed version of it? Will people live too long and die of boredom? Will boredom become the prime evil?
Thinking about it, anything that makes the civilians in CSM see death as something favourable will make it weaker.
Are War & Chainsaw scary enough to make one prefer death? War should make the corpses revive and make them continuously got at war with each other, only way they will start seeing death as something positive and long for it to weaken it.
Maybe Future devil could have a part to play and send their consciousness back in time. Or reveal them the future so they can perceive their present as a repetition?
Overused trope, but possible way to defeat Death and then decide how much to weaken it.