There is no logical reason for a surrender timer in the first place since it takes a strong majority 4 out of 5 to surrender anyway. If a team comes to the conclusion that their is no hope and they do not want to continue than that is their decision.
Its funny that the one thing Riot wants to stick their heads in and weigh in on is judging players game when they are not there. There are 10 people in the match and none of them are Riot. They are there. They know the situation. It is their choice to choose to surrender or not. Games are decided these days well before the 20min mark in most cases.
In my experience if someone starts a surrender, its because of 2 situations. Either A. Its just some QQ kid who got beat in lane, who has the maturity of a child and has his panties in a bunch and doesn't want to play anymore. Or B. The game is pretty much over and we don't want to waste 8+ minutes sitting around as the other team slowly pushes objectives, or much more likely just farms kills for the next 15min.
Now sure there would have to be some sort of rule like if you game isnt at least X long, you do not get IP for the game. Otherwise yes people could farm LP in 5min with a group of 4+ people.
You would also have to say if you are a 4 man team you need a unanimous decision to surrender for that to take place. Something you deal with if you decide you want to queue as a 4 man.
I mean look at ARAM or other games modes with no competitive point to begin with. You still have to wait 15min to surrender. There are tons of AFKs in ARAM and depending on RNG you could have a terrible team vs. a great team. However in ARAM the goal is always kills first. No one likes dieing 8+ times in 15min while the other team just gladly focuses on kills.
Heck Riot is not even smart enough to code in an escape into the surrender system that immediately cancels the surrender if 2+ people have already said no. It waits for the timeout or for everyone to submit an answer. Honestly some of the coding over there is the laziest stuff I have ever seen.