To begin with, the Past Self has transcended nine Universe Tribulations, the Future Self resides in the Destiny Realm, while he himself is merely in the so-called Sacrifice Mansion Realm. Whether his current cultivation can withstand the impact of the myriad universes upon breaking the seal remains an unknown.
The Three Lives Technique centers on his present self; the Past and Future Selves will inevitably vanish in the future, frozen in time, and only he can truly depart from this starry universe.
Furthermore, even if he could leave, what of his people?
Chapter 1018