>>149268708 (OP)The left side are all protagonists so it's best to tell the story using them.
The right side's non-protagonist do not need to be a life lesson the as that would detract from the protagonists' story.
Now for the protagonist on the right side.
Korra was written with one season in mind but after each season a new one got greenlit. For each season feel worth it a substantial goal had to be reach that wasn't originally planned.
Luz was prematurely cancelled but given a few additional episodes to wrap up the show so the writer gave everything they got for what would be the show's finally.
Star was given everything to please the fans since it was the last season. If it were to continue the creator said Marco and Star might break up and date other people since that would be more realistic.
Adora is just a wish fulfillment self-insert.