It give creators more to play with if they keep her on as a main character, even if she doesn't show up with the Mane 6 all the time. The nugget about SG being as powerful as Twilight and having SG study the magic of friendship with her means that there is a potential that Celestia herself will meet SG, then whisk her off to become a student, maybe an alicorn princess, who knows. Each of the princesses have learned some sort of life lesson before they became princesses or experienced royal-worthy problems (sisters), so she already fits that part of the bill.
In regards to writing AND marketing, Hasbro is gonna need some new material. I LOVE the Mane 6, but it's gonna get old if there are no new consistent characters. (Unless they really start following the Cutie Mark Crusaders, but they'd probably have to age them up a bit for them to be more relatable, and less "Awwwhh"-able to ALL audiences.) That's just a writer's viewpoint. Look at other current hit shows. After season 6 or 7, creators try to add new characters to keep things fresh. It either works or it doesn't.
After all of the points listed, I think they will def want to keep her as a main character, but somehow respect the Mane 6's history, or they risk alienating part of the audience.
Obvi like in real life, if you have a group of friends then suddenly you want to add one, there will always be that history... but new memories too! So I think it will be good no matter what happens... they haven't failed us yet!