Riot has no way of controlling how the players play. If they had a poor showing, that is not on Riot. If they got beat in lane, that's not really always even on them, sometimes they got counter picked and don't realize it, or the other person is just objectively better, or RNG just didn't go there way.
That would be like blaming the NFL for how the Vikings always choke every year at crunch time. When in fact they, as a team, just don't know how to handle pressure. Sadly...
Yes there will be games you can't win no matter how hard you carry personally. I experience this daily as a support. If non of my carries do well, there is very little I can do.
But it is not Riot's fault that they get beat unless Riot is matching the player with people who are plat5 and while they are only silver4 or something.
Toxicity? That falls squarely on the one who purveys it. If nothing else, solely because you signed a contract saying that it does when you first started this game up. But also because the player, themselves, chose not to move on and/or be mature enough to
above that.
TL;DR Riot doesn't control how a player plays. Just as Riot can't control how you maturely or childishly people respond back to them.