Surrender at 15 will just increase the toxicity
Admit this fact: When you surrender, it's not because the game is unwinnable you surrender because you don't want to play the game anymore. You don't want to try anymore, and you're starting to be pissed off by the game, by the enemy champ, or w/e so hard that you want to get out of the game.
Stop saying that the game is "unwinable"
As much as you play the game, there is always a chance to win, wether the chance is 0.01% or 0.0000001% there is STILL a chance to win, and this chance depends on YOU, depends if you can catch the arogant enemy adc alone, or get baron/towers if the enemy team died. Depends on YOUR itemization, if you try to get
against
instead of spamming "ff i'm afk" you will increase this chance by a thousand billion time.
Yes it may be hard to win with 3 inhib down, but are you going to tell me that it never happened to you? because my memory still have the ability to remember a game where i was afk for 15 min as jungle lee, and then our team managed to win with 3 inhib and 2 nexus towers down. My memory still have this game 1 month ago where as Lee support i won while my nexus had 10% hp, because our
always one-shoted the enemy 24 kills 
Surrendering is one of the thing that increases toxicity in league, if surrender wasn't here people would try so hard to win that the game would be more interesting, you learn more from your mistakes, you experiment more things (i shouldn't let nasus go to late game next time), but if you give them so possibility of leaving then what's the point of playing? above diamond 1 (or everywhere) it's always the same: -play the game for 10 min or 15 min -manage to understand if the game is "winable" or not, then harass your team for surrender
and don't let me start about riot giving us a surrender at 15 option, the last dude who refuses to surrender (in that case me) will always be bullied by his team to surrend. i DON'T WANT to surrender, why?
because i will always enjoy the game, unless there is a
oneshoting me from 550 range (in which case i can agree to surrender), i'll always find a way to have fun in the game, even by dying.
I play
goes 0/10 in laning phase because the jungler camped me, i simply DON'T FUCKING CARE. i will always have fun with morde, with his entire kit and maybe manage to come back because of his monstrous ability to farm, and because there is always a chance to win
stop harassing me to surrender because i'll not, and you have to respect my choise and troll/feed
TL DR: Surrender at 15 will just make people trolling because someone refused to surrender, and ruin the experience of this last player.