Major offenses (Death threats, racism, etc.): 10-year account suspension > indefinite account suspension
League of Legends has not been around for 10 years. You are advocating that a day 1 account that was unaware of the zero tolerance policy towards those things should still be banned today.
Further, what you encourage with longer bans is people just making a new account to avoid the ban. Riot could track that, yes, but it would take a lot of manpower - computers can generally determine if two people aren't the same, but it's really hard to determine if two accounts are the same.
Someone gets banned for a week? They may not even notice it. A month? They'll just start playing on their smurf. 2 weeks? They'll probably notice and not be arsed to level a new account just for 2 weeks.