Riot has said that the percent of players who reform after each punishment tier steadily goes down - i.e. of people that get a 10 game chat restriction, few may move on to get a 25 game chat restriction, but of those that do get a 25 game chat restriction, statistically they're more likely to get a 14 day ban, etc.
Basically, my understanding is Riot's internal statistics show that it's more likely to get a permanent ban after a 14 day ban than it is to get a 14 day ban after a 25 game chat restriction - but this is more a factor of people's unwillingness to reform, and not anything about the system itself.