User blog comment:Raybat/How come Death is dead but the witch lives?/@comment-79.181.5.199-20160430122020

Death's Scythe is a weapon connected to the horseman Death itself. It can kill literally any living creature there is, even God himself. Dean killed Death with the scythe because Death claimed he will kill Sam if Dean won't, and Dean refused to let that happen. Probably because the weapon is connected to Death, it crumbled as well. The 4 horsemen are symbolic, and they CAN be killed as we see, but what they symbolize must remain. That's why a new Death popped up after Dean killed the former.

Now from the point of view of the writers:

Death is too OP, and if he was still alive, he could make season 11 alot less claimatic, by simply calling God quickly, or using the Scythe against Amara. By killing Death, the writers made it seem like the Winchesters have no way to beat Amara, for now. The writers could not go on without Death, as he is a symbolic and important  horseman, so they created a new one to replace him. The new Death, however, does not like Dean and Sam, and never had any of the adventures the former Death had with them in the previous seasons. Because of that, this new Death does not like them, and is not going to help them much like the former Death.

Kills in this show aren't based ONLY on how strong each creature is, they also relate to how that creature is important to the future of the show. Rowena is a new character, mother of Crowley and a strong witch that can decipher the book of the damned. Death, on the other hand, (The one that Dean killed) had his run. He already was a big part of the plots ever since season 5, and it might be  a time for a change.