I wouldn't argue that tropes can be overused and Allagans may be one of those. At the same time we still need to pull threads left at Endwalker. This would largely be some of those threads. "Amon" had to be doing something while Emet and Elidibus were still around. We need some reason beyond prep for a later expansion to have been dealing with the 13th and this gives us some connection to that. Saying that a group was using energy from the void and we shut it down, so now they are forced to use cerullium would work. Can also assume that perhaps the stuff is like mako from ff7. Since we assume that some of the ironworks teck in the twinning was powered that way. So perhaps the soul crystal for viper is a solid form of cerullium and they make soul batteries from it. The Eliminator is a test bed for them. With smaller ones or clones becoming the new antagonist army.

I just can't see starting an expansion story from only a letter we see for less than a patch as a viable thing to do. Cutting off all the other threads just wouldn't make sense.