This isn't even touching the issue - In LoL, DotA and any other MOBA or team based PvP game with a solo queue, you can find toxicity up into the highest spheres of Diamond/High Elo and down in the lowest pits of bronze/Low Elo, despite having a multiple times tighter matchmaking than here, because it's entirely unrelated to skill, skill differentials and anything of the kind. You can be a top 2% player among other top 2% players, miss only a single skillshot in the entire game at the 30 minute mark and then get flamed because the enemy capitalizes in this and you lose.

The root of the issue are three simple things:
1) People don't care about their teammates and their feelings, since they're just randoms
2) People care too much about their precious ego and rating/themselves and get stressed
3) Many aggravated people do not have the ability to deal with their anger responsibly

It's that simple. You can't change point 1, you can't change point 3.
So if you want to solve it by the roots, you have two options:
Either you need to make people care as little as possible about the outcome of a match and having a ranked mode at all is counterproductive, because it makes people care more.
Or you need to prevent people playing with randoms and only allow Light Party queue.
(Though requiring a positive psychological expert opinion on anger management from players as prerequisite for ranked would also a theoretical option)

Neither of them are desired by the PvP community nor SE. And that's why SE is struggling for solutions. And it will keep struggling until people acknowledge ranked as a mistake and approve of the appropriate steps. That's small scale politics for you.