Cast & Crew are what help you learn about a movie, which is why we feel it should be the first section after the main section.
I can understand why you might feel that way, but I don't really think it works out that way when you consider the way we interface with the program.
The synopsis is what helps me learn about a movie. If I'm interested in learning more about a specific actor, then I will scroll down. Them being IN the movie doesn't help me learn more ABOUT the movie, and clicking on them only tells me more about the actor and their other roles, not more about that particular movie in itself.
Why is Addtional Parts above cast and crew then? The only thing additional parts tells me is to explain why the Runtime at the top of the screen is wrong...because there are additional parts.
The true crux of the issue is that there is no visual indication that a movie has Extras, unless you go out of your way to seek it out. Literally EVERY movie has a cast and crew. It is expected and known to the user that it will be there. Not every movie in a collection, and for some collections very few movies, will have extras, so it would make the most sense to highlight those things in some way when they are available. Reordering the display is the simplest way to do that. Otherwise, adding some visual indicator would be helpful, but less preferred.
You also have to consider that you have a lot more 'users' accessing Emby than you do 'server admins'. I'm not the only person accessing my library, so while I might know exactly which movies have extras (which I don't), that doesn't mean my users know which have extras. You would be surprised how many people, particularly older folks like my mom, would never even think to scroll down past the cast to look for extras. If I have to scroll past the cast and crew EVERY time to see if a movie has extras, and 95% of the time there aren't any, eventually I'm going to stop even looking for them. Conversly, if they were immediately noticeable right under the synopsis, I would ALWAYS know which movies had extras without any additional clicks. Only movies that actually HAVE extras will show the extras line, otherwise it is hidden and the cast is there like you see now.