For an example of our limitations as humans, there’s only 50ish unique plot lines in the English language.

How would the unique plotlines be determined by the language they're told in? Why would the amount of plotlines be based on human cognitive capabilities? None of this makes sense.

Either way, "unique plotline" doesn't mean anything, from the perspective of literary or narrative studies. There's no universal, objective way to dissect narratives, and they cannot be boiled down to a distinct number of basic models. There have been attempts to get to the most fundamental narrative model (Greimas, Campbell), but they're far from widely accepted.

People who are just flat against any and all AI or LLMs are behind the curve.

Art is, by itself, not something that has "the curve". If you're doing something with very practical goals and need hyperproduction, sure, but art is not necessarily made or consumed with such a logic.