First, the Past Self has survived nine Heaven and Earth Tribulations, the Future Self resides in the Destiny Realm, while he himself is merely in what is known as the Sacrifice Mansion Realm. Whether he can currently break the seal and travel to the Myriad Universes, and whether his own cultivation can withstand the impact of those myriad universes, remains an unknown variable.
The Three Lives Technique centers on his current self; the Past and Future Selves will inevitably vanish, frozen in their respective timelines, and only he himself can truly depart from this Starry Universe.
Second, even if he were able to leave this Starry Universe, what about his people?
Chapter 1018