It's a defining part of Fremen culture and religion.
It's the cornerstone of the Bene Gesserit and the Kwisatz Haderach project.
It's a major political tool for the Great Houses.
It's a core aspect of Tleilaxu society.
It's the centerpiece of Leto II's enforcement apparatus and religion.
It's a major plot point in the last parts of the saga through the Honored Matres.
Gender is one of THE most central themes in the entire series. A lot of the academic literature on Dune is focusing on it, why it's important, how it's important, etc. etc.
That's not to say you couldn't or shouldn't change the gender of some characters around, but that needs care and understanding and can't just be done arbitrarily. Kynes feels like a suboptimal choice there. More interesting, in my opinion, would have been a female Thufir or even a female Rabban. But that's just me.
It's not going to make or break things in the end, but I still feel it's not a showcase for the producers' thorough understanding of the source material. Which, fair enough, they don't necessarily need to have. Box office is mostly about boom boom bang bang anyway.