Make the Hard difficulty scale to the players level. Keep the enemies being around 3 levels higher than the player for hard instead of a fixed level. Outleveling the difficulty makes it trivial, hard is suppose to provide a challenge.
I think the MMORPG aspect of Destiny is why this is done this way. Some levels are meant to be done at certain points in your progression. The "hard" difficulty is for when you are a lower level. I imagine there will be plenty if challenging missions later for you choose Hard difficulty that you won't need to worry about scaling of lower levels.