People would still use smurf accounts to help their friends rank up. People would never have to lose any lp that way. They would either win their matches, or have the player on the disposable account afk and take the hit, and they would get loss prevented. So they would never actually demote in rank, and the entire rank system would get skewed horribly.
It's an unfortunate problem with relying on other players in a multiplayer game. Sometimes, you will have players who leave when you need them. Equally unfortunate is that many of the suggestions people have for addressing the issue are too easily exploited to be effective. But Riot does still look at this situation, and does consider all the options when looking for ways to punish the players who make the games so difficult for their teams by leaving, without causing an even bigger problem, or risk accidentally punishing innocent players.
One of the ways they have implemented rather recently was the Low Priority Queue, which does seem to be fairly effective at punishing people who leave games. Though some think it might be a bit overly harsh, having multiple games with a 20 minute wait before queuing likely does deter quite a few people from continuing to afk. And the good thing about how LPQ works, is that no one is given the five 20 minute waits for a first or second offense. So, it does allow for emergency situations, or things that are out of a player's control, like a family situation or a sudden power outage. If a player does not show a pattern of this sort of behavior, or a sudden escalation of it, they are not likely to face any sort of serious inconvenience. After all, it's only meant to be a deterrent to discourage people from leaving games, not a way to keep them from playing at all.