Look at it this way: only 0.006% of players ever get permabanned. If you are so utterly unpleasant to play with that only five more players out of every 1,000 people are as bad as you, why should Riot lenghten the period that you can create a bad experiece of the rest of the players? Before a permaban you are already hit with 3 other scaling punishments, and if you're still haven't got your act together, then you probably won't ever get it together anyway. A permaban is extemely rare, so if it's even a possibility for you, then that means you're just not the kind of person that Riot wants to have as customer.