Major thing holding spike rush back is that you can't choose your gun, always having to play full shotgun/sniper rounds and so on. Replication on the other hand has an amazing gun progression where you can choose how you want to play and, in my opinion, salvages an otherwise annoying game mode that usually features 5x sage walls or raze nades.

It would just make too much sense to put 5 of a kind guns into Replication and spike rush to have freedom in gun choices.