I love seeing the number of people that mix up primary and special ammo weapons with Kinetic and energy. Two different things. To answer your question, I would venture to say it is to balance weapon loadout choices. One way or another, something was going to be different. Weapons in the kinetic slot will always do kinetic damage, hence the name. Meaning that something like Vision of Confluence would be kinetic rather than solar if it was in the kinetic slot. Closest they could have come to original VoG weapons is if all primary weapons went in the kinetic slot. Meaning that just like how Fatebringer is kinetic rather than arc, Vision would be kinetic rather than solar. It would be different somehow always because the kinetic slot will always be kinetic damage and nothing else.