You should lose less LP than normal if you have an AFK on your team

instead of losing 17-20 LP for losing a match with an AFK, why not lose just like 20% less LP instead... It makes sense

Edit: If people would leave the game just to take advantage of this, you can always create specifics to this, such as being AFK for 20 minutes or more. That way people can't go afk last minute just to lose less LP.