Players should allow to leave without penalty when another person quits
Playing a 4 vs 5 is already a negative experience in most games as the team of 4 has little chance of winning assuming the teams are near equally skilled.
It therefore makes little sense to force teams of 4 players to continue playing a game to a minimum of 15 minutes to forfeit (could be significantly more if someone refuses to forfeit) given it only furthers the negative game-play experience. This current system effectively keeps the team of 4 hostage as they are forced to play it out and cannot leave an unfair and unfun game as they would also suffer leaver penalties and this only creates unnecessary frustration due to powerlessness against leavers.
I feel League should adopt the Dota system in handling leavers. Their system allows players to leave without penalty only after a couple of minutes have passed after a person has not reconnected after disconnecting. This allows a buffer for legitimate disconnects to reconnect in which case the game can continue. However, if the buffer elapses, all players can leave without penalty sparing them from being trapped in a negative game play experience. The match would still be considered a loss which would preserve competitive integrity.
I'm not disputing it is possible to win 4 vs 5s or even 3 v 5s and yes every player can vote to surrender how they want but I think it is important to acknowledge that these scenarios more likely then not result in a loss for the team with leavers. If the leave occurs early in the game, this can mean playing for an extended amount of time at a significant disadvantage only to inevitably lose which can be incredibly frustrating.
Just because this option exists doesn't mean it has to be utilized. If the players think the 4 vs 5 is still winnable, they can of course continue to opt to play. It only enables players to save time when it is obvious the game is not winnable.
Continued Note: I think it is fair for the person who initially disconnected and failed to reconnect after the set time period to be punished. It is everyone's responsibility to ensure a stable internet connection when playing and a team shouldn't be penalized because of one's persons faulty internet connection.
I don't think this change would enforce a "GG at 15" mentality. It only reduces some very real frustration caused by leavers in which players have very little power in response.