With a big cast like this, the lack of development for most of the regulars, and the trend of making it the kirk show already, I understand and share some people's trepidation over them possibly shoe-horning Jaylah or another original character when the little screentime they have in a movie could, and should, be used to further develop the existing characters and their dynamics - not to try to find pretexts to give new characters something to do.
The problem with new characters is exactly that because the iconic crew is taken for granted and might get sidelined under the assumption the audience knows them already or enough, while with the new character there is an effort to make them visible and known to the audience, which may result in them being pretentioustly placed in certain scenes (eg the comics writer did that a bit with Carol in some issues where she'd be included in some moments just for the sake of reminding us of her presence and not because she had something to do, all the while you wouldn't see some of the main 7 for entire issues regardless their expertise possibly being useful for the mission at hand)
I think new characters should stay secondary and play secondary roles in context of scenes where their presence makes sense. They should be a plus that the narrative permits, but the priority for me is developing what you already have before painting yourself into a corner by adding too many characters and stories you won't be able - wont have the time - to develop.
