There has to be a new addition to the game that prevents a loss if a teammate is disconnected for more than 75% of the game or something. I mean, I just played a game where Xayah barely connected by 3 minutes, said she had multiple bug splats and wasn't actively in the game for more than 3 minutes altogether. I'm a support Soraka and I can't do anything about it. It's unfair that I have to lose LP because of situations like this one.
I'm sure you can spot a pattern for when someone rage quits and when someone keep disconnecting and reconnecting, but doesn't move his character a lot. So you could differentiate between leaving on purpose and not being able to stay connected to the game.