It's much more believable and richer than humanity simply being the Forerunners who personally regressed themselves when they needed their ingenuity the most. Such a grand species going from a Halo to a cave of their own accord is something I really doubt would ever happen. Even then, Bungie's intentions with the Human-Forerunner connection seems vague. 343 Guilty Spark speaks very strangely. He refers to John like they've met ions ago, and despite being almost entirely human, is referred to as a Forerunner. It honestly seems like 343i's geas and related structures are rooted well in Bungie's mythos for Halo.