I feel like a bigger concern for arbitration was and is the fact it is survival and defense are not worth doing compared to interception and excavation. You get more rewards quicker in excavation and interception than you would on defense and survival. Keep the modifiers for survival with halved life support on towers and capsules but rewards should be around 5 minutes like every other survival mission. Defense will always be the longest even if you reduced it back to 5 waves. Endgame shouldn't be about how long you stay in a mission is my take on it.