I believe that our guardian was meant to die while fighting the final battle against the witness. This seems evident by the fact that ergo sum or the sword of the traveler, was present on our grave in the corridor of time. Now consider the vex cannot predict paracausality, meaning they wouldn't have known that Crow would wish Cayde back to life with an Ahemkara wish. I purpose that by bringing Cayde back, Crow accidentally changed the outcome as well as the events of the final shape. The most noticeable change would have been the loss of our ghost since cayde brought it back. It's also entirely possible that without cayde, the vanguard would have fallen to the witness, thereby not being able to cover our escape once we wounded him. Just my thoughts.