It's also a way to keep the new and current content relevant, if you watch the twitch stream from today you'll notice that the replacement for the raid the arena activity is going to be a weekly event that will change from week to week and have set predictable rewards each week like Iron Banner does. Also with the addition of features like reforging weapons and upgrading existing armor and weapons, a lack of a new Raid keeps the two existing Raids relevant as you can still run them then upgrade the weapons and armor you receive to then go play story missions, Prison of Elders, and Trials of Osiris. So essentially the entire game is still relevant as any and all legendary and exotic gear and weapons are now usable in endgame content.