Kicking a player doesn't lead to more toxicity.

If anything it prevents toxicity.

Being able to kick that afk player will reduce flaming.

Being able to kick that 0/8 by 10 minute Tristana bot lane will prevent her from getting flamed.

When you have a player who only afk farms well into 30 minutes intentionally, you kick him and everyone remains in a better mood.

With the billions of dollar Riot has they should create an automated system for better AI and give people the option to kick players who obviously aren't playing ranked to win.