I've never understood this. Do people really think that's it? You achieve a goal and then you just stop growing? That just sounds lazy to me, even if you're writing characters.
Twilight becoming a Princess, or even a Mentor doesn't mean she's out of lessons to learn, Rainbow becoming a Wonderbolts doesn't mean her journey is over. Maybe the show will end soon, and maybe the writer's will end those characters stories, but that doesn't mean that HAS to happen.
If you write good enough characters, there doesn't have to be a growth "Cap" like that, it shouldn't be too unbelievable to think of other directions the characters could take even after reaching their goals, and I think the characters in this show are good enough that's possible.