>>42771529
It was only natural that Misty replaced Sunny as protagonist, because the Sunny's arc was basically over when she became that pseudo-alicorn thing and re-united the races. There is still 3 soberaign city-states in the g5 equestria, so Sunny didn't and wouldn't become a princess, like Zipp, Pipp or even Misty, as she is the heir to the leadership of Bridelwood and step-sister to Zipp and Pipp. Even if I wished to know a lot more about Sunny's backstory (how her father died why her mother also died or who was her), she couldn't keep the title of main protagonist because she hadn't a lot to give, but an analogous arc to that of Twilight about becoming an alicorn and learning to control such powers. (We are also left hanging with Misty's mother, wich is a shame).
Tltr: So I personally don't mind Misty replacing Sunny, because to me Sunny was boring as an MC.