Besides the fact that running older gear punishes you by decreasing damage while increasing incoming damage. It also reduces your rewards, making all power drops lower levels, reducing tier level, and reducing frequency of engram drops. But beyond that, unless bungie greatly increases the number of weapons dropped each expansion, it also hurts build craft. For example, here is currently only 3 special weapons that can drop in the kinetic slot (which have antichampion synergy) this means yoyr limited to running a primary in kinetic, and a special in energy. Then there is the fact that most weapons this season are solar, which limits your ammo, and orb builds as well as elemental burns for activities. In short, unless bungie wants to start ensuring each season floods us with a plethora of weapons archetypes which cover all elements, then they just need to drop this sunsetting light mechanic.