The Drifter, Osiris and the savior Guardian had all built the sundial.
Some time had passed and Saint-14 had returned.
The guardian began to wonder if he could bring Cayde-6 back from the prison of elders.
The architects (BNG) have stated He won't return and even confirms it himself within the Ace in the hole mission.
One of the only possible ways is just to let the red legion to gain control, expand the Sundial's reach of the temporal rift. This has unintended consequences they will reach the traveler they will take its light so this is not a feasible option.