The reality warping Oryx can do isn't the same as what the Precursors do. Oryx is able to manipulate that which already exists is some limited way by Taking, but cannot himself call something into existence. The Precursors can. Also between the Shield worlds, fortress worlds, and other such super weapons while the Forerunners may not be able to kill Oryx in his Throne (I think they could given their power and ambition but we'll assume they can't) they could still kill him physically with ease and reduce him to a non-threat by just killing him every time he reclaims mortal form and, for lack of a better word, spawn camp him with a small fleet for eternity.