Perma ban people for quitting? LOL

Yes, permaban people for quitting. It ruins the experience of everyone else in the game. Your teammates get thrown under the bus and are handicapped, the opponents get a free win, and it's all around just a worse game.

If you're thinking, "what if I need to go to work in the middle of a match" or something like that, learn how to manage your time better. Stop joining matches when you know you'll need to leave in the middle of them.

There is also nothing to 'grind' in MCC.

I know Halo isn't a great competitive game, but that doesn't mean you can't grind it... well, you can't because of the problems I talked about, but it's possible if those are fixed. I don't need random arbitrary rewards to grind a game, I wouldn't be playing fighting games if that were the case, I just need player progression.

Play FFA where you don't have to worry about getting teamed and can learn the general controller basics of the games.

That changes nothing, there is still one or two people in the lobby who wildly outclass everyone else. It basically just becomes a 1v1 to see who can farm the most free kills off the bad players in the lobby.