Great scotts I made a theory... Halp.
Okay, so if you know about FNAF 6, AKA Freddy's Pizza Simulator, a fire burns down the building, right? Well, somehow this reminded me of the box from FNAF 4. I think I know what's in the box. The thing to stop it all.
So, hear me out. Somebody, maybe Henry, owns this box. He knows about the sins William caused. So, he actually planned to end this whole thing at FNAF3, attempting to burn it all down. After a bit, he thought it was all over. Toys were scrapped, originals were freed. Everybody was happy, right? No. Henry learned that Springtrap survived the fire, and more animatronics are out there. So, he put the lighter back in the box till it was the right time, and when it was, he unlocked the box and ended it once and for all.
Of course, like always, I might be wrong on this theory. There are better theories out there. Tell me what you think.