Technicalities on their part. It's more of an mmo than any other genre. Point is, yes even WoW released content that makes their old content useless. Don't believe me, then you haven't downloaded any recent expansions. And seriously how boring would destiny be if everyone was still using year 1 weapons/gear. The expansions force the player to stay engaged and keep it from become stagnant. Also promotes longevity, which is something they will need for a 10 year plan.