Playing a 20+ minute ranked game as 3v5 is punishing and unfun

There needs to be a system which influences LP loss when you're out matched 4v5 or 3v5. So if you play an entire 20-40 minute game as 4v5. The player who either could not reconnect or rage quit, or what ever the reason they are gone from the game. They lose the full amount of LP (help prevent abuse), and the players who stuck through till the end lose a reduced amount, maybe half of the original LP lost. It feels punishing and unfun when you try your best to win a out-matched game. And still lose the exact amount of LP you would have if you lost naturally.

Bit of a wall of text, but it explains just one example of the struggle a friend and I recently experienced...

So, I was trying to help out a friend on my brothers Silver account. And yesterday our games were going great, we won 8 out of 10 games, my friend moved up to Bronze 1 with 75 LP and was really excited. Called it quits till "tomorrow" (today) and, well today was not our day. We seemingly hit a wall. First game we played our Shen goes top, and gets utterly pooped on by a teemo 0/3/0 within the first 5-9 minutes, rages at the jungler, and left the game. We tried our best, jungler went top and did decently. I was crushing mid, and my friend was doing alright as support. Our ADC had decent farm and there was no real sign of anyone raging or upset that it was 4v5. We were all trying our best to get ahead and start forcing team fights. The game lasted to 45 minutes and we lost because we couldn't end it soon enough. He lost 14 LP, he wasn't upset because "shit happens" and you don't get ragers every game. But...

Our second game it was our jungler, he got invaded by a Shaco and died losing his red. Shaco camps his blue and gets another kill. Going directly top and getting a 3rd kill on our Darius. Without a word our jungler left the game, and never came back. This time my friend lost 16 LP. We decided to give it a couple of hours to calm down and relax...

We queued up for our third (and sadly last) game of the day. In Dynamic Match Making Process our decided top lane, did not want top. He wanted mid and said he would troll if we didn't give him mid... As I was mid and I really don't mind going top. I said fine, go mid it's okay. He goes lucian mid vs Zed, he picked into zed... No one says anything we finish picking champs and load up in game. He starts with Dorans Ring, and 4 minutes into the game dies to zed alone. Proceeds to die 5 more times, then just stands around in fountain saying he can't do anything anyways. He would just feed more if he went back into lane... Losing another 16 LP...

I just hate feeling like I am being punished for not being able to win a 4v5... And I also think something along these lines would reduce stress and toxicity of players moving from one game to another.