We don't know exactly what level of catastrophic damage the Ark took when the Halo fired. 343 said it would destroy it, but I don't see the whole thing being vaporized so there must be [i]a lot[/i] of salvageable wreckage... that is if no Sentinels survived or weren't brought in from other installations to repair and refit. Also, how do we know the Greater Ark can't produce a Lesser Ark replacement?