First of all, fair use is not simple or as clear-cut a concept that can be applied uniformly to all cases than you make it out to be. It's flexible and context-dependent on careful analysis of four factors: the purpose and character of the use, the nature of the copyrighted work, the amount and substantiality of the portion used, and the effect of the use upon the potential market. No one factor is more important than the others, and it is possible to have a fair use defense even if you do not meet all the criteria of fair use.
Generative models create new and original works based on their weights, such as poems, stories, code, essays, songs, images, video, celebrity parodies, and more. These works may have their own artistic merit and value, and may be considered transformative uses that add new expression or meaning to the original works. Providing your own explanation on the reproduction of fireflies isn't making the original redundant nor isn't reproducing the original, so it's likely fair use. Plenty of competing works explaining the same thing exist, and they're not invalid because someone got to it first, or they're based on the same sources.
Your example about subtitling a movie doesn't meet the criteria for fair use because subtitling a movie isn't a transformative use. It doesn't add any expression or meaning, you doubly reproduce the original work in a different language, and it isn't commentary, criticism, or parody. Subtitling a movie also involves using the entire work, which again weighs against fair use. The more of the original you use, the less likely it's fair use. This might also have a negative effect on the potential market for the original, since it could reduce demand for the original or its authorized translations. Now, subtitling a short clip from a movie to illustrate a point in an educational video or a review would likely fly.
Finally, uses that can result in lost sales for already established markets tend to be determined as not fair use by the courts. This doesn't mean that uses that affect the market are unfair. That would mean you wouldn't be able to create a parody movie or use snippets of a work for a review. These can be considered a fair use because they comment on or criticize the original work, unlike uploading a full movie, song, or translated script. Though I could be getting the wrong read here, since you didn't explain how you came to any of your conclusions.
I think you're being too narrow and rigid with your interpretation of fair use, and I don't think you understand the doctrine that well. I recommend reading this article by Kit Walsh, who’s a senior staff attorney at the EFF, a digital rights group, who recently won a historic case: border guards now need a warrant to search your phone. I'd like to hear your thoughts.
