The last few weeks I've been seeing comments on the Wikia and on other sites like MLPForums, Equestria Daily, etc. and they're been saying stuff like Starlight's had her moment and let's focus on the Mane 6 or Starlight's taking over the show or Starlight's taking time off the Mane 6, even though they are a vocal group, turns out it's a small number of fans who say that sort of stuff and some even leave out Starlight in their artwork, last year's holiday artwork is one good example and anyway here are my 6 reasons why the show won't go back to the Mane 6.
1. The Changelings were one of Equestria's biggest threats but after the Crystal Empire and the Equestria main ponies spotted and saw Thorax, their mind on him had changed after Spike convinced them that he is a good changeling. He was a supporting character in the Season 6 finale and was one of the many Changelings who transformed into an ally of the ponies, Starlight would no doubt be credited for this and the ponies and the changelings would be working together in the future, even Thorax himself had no idea what to do because he was out of the hive for so long, he no longer had knowledge of the directions of the hive.
2. Even though she had a major role in the two parters in Season 6, she hasn't completed her friendship lessons with Twilight and she will have to continue learning and developing as a better pony to all of them from Twilight and just putting less focus on Starlight would make her development incomplete which would disappoint many fans.
3. Somewhere in Season 7, presumably the premiere, Starlight will probably be honored by Celestia and the princesses for rescuing all the most important ponies in Equestria, I doubt that the ponies will ignore what she did without some sort of honor or reward from them and before you say, the Mane 6 save Equestria lots of times and defeated many villains, well they are the main heroes of Equestria so this was expected from them.
4. You have to remember that Queen Chrysalis is still on the loose and Equestria isn't quite safe yet and who knows what sort of plan does she have, Starlight may have to deal with Chrysalis herself as she was partially the one who destroyed her kingdom but leaving Starlight on her own and letting her deal with Chrysalis herself would make them a very bad friend to her and that would ruin the show's reputation.
5. Going back to the Mane 6 would make a very awkward situation, it would mean that they would go back to what they were doing and pretend what happened to them in the finale never happened to them, if anything, the ponies would have to be more cautious now that they were captured by Chrysalis and they had no knowledge (except for Luna) until after they were freed. The ponies will have to be more careful with what they do now and watch their back at most times.
6. The show just released a new logo sometime last year or this year as seen in this picture here https://twitter.com/TRMWilliam/status/814831354571530240 and normally whenever a new logo is released, there's probably some big change or surprise that we may not know yet and assuming that none of the writers or voice actors answered any questions regarding Season 7 on sites like Twitter then that would make the fans more anxious.
So there are my reasons about Starlight, I might have more but you kind of get what I am saying, if the show goes back to what it was pre season 6 then it would seem like the events of Season 6 never happened and seems a bit unrealistic to me. Now I'm not saying that she will replace the Mane 6 in the show, that will never happen, we'll always see them doing what they do but what I'm saying is maybe they'll be a transition of a central character from Twilight to Starlight, even before Starlight reformed Twilight already had a decreased role compared to early seasons when she was a unicorn.
Well, that's me done talking.