I know Wu said he caused the Merge, but what if it was really Nya? Not directly, but what if her actions were the catalyst that eventually resulted in Wu needing to cause the Merge in order to save all 16 realms?
Back in Season 5, Nya used her powers to destroy the Preeminent, the physical manifestation of the Cursed Realm. Then in Season 6, Djinjago is shown to be falling apart. As Khanjikhan says, "The Sixteen Realms are interconnected and the balance affects us all. When one falls, another falls apart. Every action has a consequence."
Even though most of the events of Season 6 were erased, nothing would have stopped Djinjago from being destroyed (and Dragons Rising shows us that it was). But did Djinjago have another realm it was connected to, other than the Cursed Realm? And did it start a chain reaction, impacting all of the other Realms in turn?
What if all 16 realms were interconnected? According to the Ninjago wiki, we still don't know three of them. What if they are already destroyed, and Sensei Wu had to cause the Merge so that "united" those remaining could stand together?
What if Wu's two Wojira empowered students came the closest to destroying Ninjago? Morro by allowing the Cursed Realm access, and Nya by destroying the Cursed Realm and inadvertently impacting of the others as well?
















