Most of the story is yet to come, we've just had an introduction we don't know what the vex the hive or the fallen are or why they're here we don't have the faintest clue what the nine are we don't know what Rasputin did we don't know what the darkness is we don't know what the traveller is we don't know who the stranger is or what she's doing what the awoken are doing where on Earth the city is what year it is but in future missions and explorations and discoveries we will find out! there are mysteries all around in this game. even the maps you walk in, the buildings you see, have their own mysteries, eventually stories the game is steeped in mystery they've whetted our appetite, so when we get answers it will be so much more satisfying or when one if these races invade earth/the city and we have to fight them off

It's all about appealing to the masses. The core mechanic behind Destiny is content repetition. Once you have "found the intelligence" Stole the secret bla bla bla what next ? Having the vex reseed an area after you've wiped the out makes as much sense as much of Destiny. Destiny lore has 5 clans. Guardians - walking zombies can not die. Vex - mass produced machines infinite Hive - insect race affectively infinite Cabal - Clones Fallen - only possible limited lifespan race. Having a world that recedes itself makes sense with these races. I believe Bungie saw the hardcore mmo players replaying content and saw halo players replaying their favourite levels and thought if we add random loot they get rewards in addition to replaying the levels and will love it. The Japanese RPGs seem to do this all the time. (Dark Souls, Dragons Dogma etc.)