In the previous simulation, he had neglected one crucial matter.
In the simulation, he possessed the Doomsday panel and could initiate Descent of the Ghost God without the need for Space-Time Summoning. Paired with the "Imitator", he could trigger Descent of the Ghost God twice in rapid succession, subduing the Samsara Saint and the Behemoth one after another.
But in reality, he needed to activate Space-Time Summoning before he could trigger Descent of the Ghost God. If the Space-time Replacement was interrupted, then he would need two "Imitators" to mimic Space-time Replacement and Descent of the Ghost God separately to complete his mission.
However, he didn't have two "Imitators", and the Dawn Apostle would destroy the Teleportation Stele the next day, cramming a massive number of people behind the Pure Land Gate.
If he couldn't reach the Pure Land Gate before the Space-time Replacement ended...