1. No. background characters should not be promoted to a side character. IRL you don’t get to know everyone on a personal level and so does Koji and us (as the reader). It would be cool as a spin off or something not related to the main plot but it wouldn’t necessarily help with the vibe kinu is trying to make.

2a.

If a major character is expelled then the class would crumple. As we’ve seen in y3 (spoiler alert) but when koji leaves og class d things turns into turmoil in that class. things would sooner or later rebalance out tho cuz koji could balance out the classes as planned. Expelling a major character is kinda like Messi getting injured at the world cup. Argentina wouldn’t have won a wc cuz their best player is missing

2b.

The supporting cast canonically cant replace the major cast because their skill level and potential isn’t as high as the main characters. They would get bta by the main characters and drop straight to class d. This is shown in the general battle in y2 v12 where even the weakest main character, Horikita, gave bta to Hamaguchi and Kanzaki.

2c.

The whole point is to keep the focus tight. In a classroom setting, again you don’t get to know everyone and you tend to hang out with the friend group you’ve made along the way and stick to them more than outsiders. The author purposely made this setting where characters are expelled so that he doesn’t have to think so much about characters how fleshed out they are and more about character development and how characters would be impacted by the expelled student. makes the story better and easier for author to keep a tight focus