I do not agree with this. There are too many negatives, but we can discuss this if you want. Maybe you can find a better solution.
A. Is a 9/0/12 toplaner good when you lose the game?
Absolutely not. He is the worst teammate. If he had 0 deaths and you were losing, it means he was not teamfighting at all. And yes he may have had pressure splitpushing potentially, but if your base is about to be taken you better believe he should be there fighting.
B. Do you want teammates to just afk farm instead of trying to win?
Grade largely comes down to cs. If you are losing a game in order to save your cs you just start farming waves and just stop fighting. Well guess what, now your team is almost guaranteed to lose.
(There is farming waves in order to fight later, and then there is farming waves to just be afk).
Ultimately it means once you start losing the game, you stop trying to win and start trying to save your LP. Which is bad.
I will say however. In low ELO, I think Riot can make a system where you never derank or lose LP.
Lp and rank are imaginary currency. They literally don't matter. It doesn't matter if there are 1000 S5 or 20 S5. So the system can be in place to help people climb, some faster than others but with less punishment for losing. Because losing is a normal thing.
This system would need a lot of tweking to be fair fun etc. But I think (other than I dislike participation prizes) it would just be a less stressful environment, which means more fun for more people.
And higher ELO can continue to climb as current.
If anything the worst thing is people can't make fun of Bronze5 anymore if no one is bronze5. So it just turns into blanket low ELO. But people should just grow up.