I didn't knew about the long term writers focusing on Starlight. I admit I overlooked that fact. That said, I think the focus on Starlight occured so she could become more regular character. Mane 6, CMC, Apple family, Discord etc are pretty much established in the show so what we can assume is they needed to focus on her bit more so the watchers would get more familiar with her and so she would feel more legit.



Think about how prominent elements of Harmony were for first two seasons. How important Key episodes were for season 4. How prominent map episodes were for season 5. What I'm getting at, is that in essence Starlight is an idea that was heavily exposed in paticular season, so the watchers could get used to her, and was used as a plot device in a way.



Will we see Starlight in season 7? of course, but if you see show's history you'll notice a pattern of Mcguffins that serve paticular purpose in one season only to step aside to give room for next one in next season and taking that into account I think it's very possible that the focus on Starlight will be lessened in s7. She will belong to regular cast, yeah, but personally, I don't expect 5 or more episodes focused on her next time. That's the other way to look at it. All you need is to see how previous seasons were crafted to see that pattern. the only difference now is that this time the Mcguffin in question is actually a character, not inanimated object so the handling had to be bit different, but the pattern is still there.