Well, I'm not sure that is what is at issue here. The reason that this is getting backlash is because one of the most prevalent themes of Lord of the Flies is the consequence and toxicity of hyper-masculinity.
That's a somewhat narrow reading of the book's content, and not one I've seen many make, to be honest.
There are several, much broader overarching themes in the book, including some almost directly referenced within the text that aren't necessarily tied to gender. Society, innocence, evil, etc, etc.
And even if that were a major theme of the book and I suppose it could be argued that it is...so?
This is a movie adaption, and a change is clearly being made. That means the nature of the themes could change as well.
The writers say that they intend to make a very "faithful" and "contemporary" telling of this story. But it is difficult to do that when what is quite literally the key theme is nearly impossible to explore with swapped genders.
Which key theme is impossible to explore? Toxic masculinity?
It's unlikely that it would be explored in exactly the same way, but impossible? I wouldn't say so. That theme intersects with others, and we could see something fairly similar to the original.
I dunno, I would think it might be interesting to see "women acting like men" in a sense.
So yeah, I don't think a lot of the objections are necessarily about the overt acts in the books. Of course women can do horrible things to one another. Of course women can build a failed society. But if the key theme of the story cannot be explored in the context of women doing these things, then you are no longer adapting Lord of the Flies (much less making a "faithful" adaptation). At that point you are telling an entirely new story.
So it's an entirely new story. Why can't it be explored in the context of women doing these things?