Riot's main argument against handing out Loss Forgiven when you have a leaver isn't because of people abusing the mechanic. They believe that instituting that system would incentivize players to bully underperforming allies into leaving, in an attempt to save their rating.
The situation they envision looks like this:
Your mid is feeding pretty bad, and it's probably going to snowball into a loss. You're in your promo's, and this will result in a failed series. If he leaves, you get another shot. So, you try and MAKE him leave.
Would that actually happen? Maybe. Given my experience with this playerbase, it wouldn't surprise me at all, honestly. In any event, this is Riot's big problem with said system. It could lead to more toxic interactions between players.
The problem isn't "Loss Forgiven" for a leaver. The problem is
"Hey guys, we know your game is logged as having only 4 people connected since 00:00, but you're not at any disadvantage and it will be a fair match."