That's not how it needs to work.
The faster a team wins, the more dominant they were.
The slower a team loses, the less they lost by.
So to reflect this, LP gains and losses need to factor game time in. A FF@20 game should reward the winners (and cost the losers) more LP than a 60 minute game. This kind of scenario would also discourage surrenders.
Unless the negative transfer system were implemented for games with an AFK. The negative transfer would transfer all LP losses to the AFK player, so this kind of system would encourage the players to heavily punish the AFK by surrendering at 20 to ensure maximum LP loss for the AFK player without risk to their own LP.