By deciding to keep Alia Atreides (Pauls sister) unborn it drastically shortened the time frame they spend with the Fremen, getting immersed in their culture and gaining their trust/support, from years to to months. That means everything feels rushed and they also e.g. cut the death of Paul and Chani’s first child. But then again this change was probably for practical reasons, because the movie was already long as is and it also avoided having to cast a child actor. ___