lost LP along with MMR despite the fact that it had nothing to do with me or my teamates.
You win as a team, you lose as a team. That's the way it's always been, and that's the way it will always be. If anything were put in place to affect LP losses or gains other than winning or losing games, than individual ranks and the ladder as a whole get polluted.
Would you also be fine with gaining 0LP if the enemy team had your troll? I doubt it.
Your rank is determined in the long run. A few AFKs over the course of the season is not enough to significantly affect your rank. If you're not climbing, you need to adjust what you're doing. Also keep in mind that the enemy team has a far greater chance of having the troll/afk as long as you never do those things. 4/5 on your team, 5/5 on theirs.