Arguably the toxic player base is riots fault. When they put snowball runes into the game and made comebacks near impossible, what did they think was gonna happen? Granted yes, people are all responsible for their own actions, and should be held to account for them, but so should Riot for creating this mess that is the game now.
When losing feels like having someone rip off your head and shit down your neck for 15-20 min with little hope of escape because your plucky teammates and their "never surrender" attitude (which don't get me wrong, I would have found endearing last season, when their WAS hope of a comeback) even after going 0/10/0 botlane when even just one of those deaths can cost you the game, its not exactly surprising that even the best of people are succumbing to toxicity now. Riot can't ban them all because they'd lose most of the remaining players if they did that to everyone that had a game where they lost their temper and got toxic. Again, not that I'm defending them, just saying I can understand why they do it.
Another thing is the fact that inters still aren't punished properly because "they could be just having a bad game". A smart company would have programming to watch for people going on long streaks with seriously negative KDAs (-8 -10, somewhere in that range), and if it goes on too long, they should get suspended.
Its one thing to have a bad game, or even couple bad games, but because of the MMR system, if you do poorly for awhile eventually you should drop down to where you will do better if you aren't doing poorly on purpose. Could inters get around this sort of system? Probably, but if it's designed well enough they will have to at least do well in some games, and that's games they aren't ruining for 4 other people, and it'd be worth it in my book.
If they fix anything though, it should be the snowball. I think you'd see ALOT of the toxicity disappear if games weren't so damn stressful becuase one mistake can cost you the game (and 15-30 minutes of your life down the drain).