Like we saw throughout the show, Twilight has a great deal of hyper dependence derived from Celestia that is never, EVER resolved in the show and holds her back in complex situations like how she sabotaged things just as much, if not more, as she fixed things through the theatrical movie.
Remember how out of touch Twilight was when trying to get Starlight to get random people to be friends with instead of Trixie? And the level of desperation Celestia noted from Twilight throughout that episode, and the doubtful look on Celestia’s?
That episode, Celestia finally had the face of Hank Hill thinking “that boy ain’t right”
If Celestia kept that skeptic face she had when it was clear Twilight was so immature she would throw all friendship lessons out the window just to please her, she would not have been made Celestia’s replacement.
But, her being Celestia’s replacement was decided at the end of S3, Twilight has no major conflicts with Celestia, her princess hood, or twists and turns in her life that forced her to grow internal wisdom.
That simply didn’t happen with Twi. Not only is she hyper dependent, she has one of the most linear of ‘main characters’.
AJ, Rainbow Dash and Twilight all have qualities combined that make good leadership, while Pinkie Pie, Fluttershy and Rarity are the most in touch with the smaller details of any situation. Spike acts with both intense curiosity AND a grounded attitude(mature for his age having to deal with Twilight’s insanity), and in the other group, Sunset overexcels in the assertiveness Sci-Twi lacks.
They are a perfect team, often very underutilized.
The show rarely incorporated the 6-7 as a group very well as the series went on. Instead of increasing the stakes, they often shifted the focus instead.