Considering Riot suggests for banned players to create a new account, they don't really want people to leave the game they want people to farm up new accounts and hopefully buy more into the microtransactions. The almighty dollar reigns supreme, if they were serious about removing toxic people from the community support wouldn't openly suggest that a toxic banned player create a new account, they would ban them from the game completely and stop them from creating a new account.
They do the exact opposite which almost encourages negative behavior. They simply use the permaban as a gesture of goodwill to the community whereas it's just them trying to make more money on the game which makes perfect sense in a business mindset. They really don't care for actually making a system that works, sort of like they really don't care about actually balancing the game which is why the meta changes (the fact a meta even exists) and new champs get pushed out more and more, instead of Riot slowing down with dishing out new champs and working on balance, the cycle continues.