TLDR; There are MANY factors you're overlooking in this suggestion, and it would be a bad idea to implement the system without addressing these issues because it simply would not be likely to work or solve the problem.
So I think two things are true, the percent of players who are toxic is really small. By this I mean the number of players who are consistently rude or toxic. Riot has acknowledged (at least I think I remember reading something) that much of the negative behavior is actually from consistently positive players who have a bad day or a bad game. I've had ribbons on and off for most of the last year and pretty much only play ranked. (This probably puts me higher up in terms of "positive" behavior since riot has talked about the issue with ribbons and how they are skewed toward people who predominantly play bots, so being on the borderline of ribbon means most games I'm probably pleasant.) However, fro time to time I have a bad game and would report myself for the behavior.
Now I'm not saying that the idea CAN'T work. However, finding the right balance will be really difficult or impossible. If you make it easy or a low number of games to earn it the riot would be stupid to implement it because the will lose money giving out RP material for free frequently, and the community behavior will not improve. I.E. I'm definitely not enjoyable in some games, but am positive overall, if I can earn incentives still doing this why would I change. On the other hand if you make it to hard people won't work for it and again behavior will stay the same.
To give an example of the second, ribbons are like top 20% of "positive" players or something like that I think? If you have to keep ribbon status for the entire year to get a skin many people won't make it and will give up after they miss the requirement. If you have to maintain it for a a certain time then maybe people improve, but then the requirement of top x% becomes stricter and people will likely stop working as hard at it because they think they can't get to it. Now the argument may be "but top x% is too hard to manage to do, so it would be lower requirement. Like positive for x games or something that is standard rather than having to do more than your peers." I'd say the argument against this is above. If it is just x games, then I can be positive for x games, then negative for x games, then go back to positive. If 100k players are in this same pattern then it only takes 10% following this pattern to have someone negative in every game.
Additionally, and I mean this as constructive criticism not an insult, probably 3 or 4 out of every 5 games I play my whole team is pretty chill. 1 in 5 someone is a jerk and 9 out of 10 of those are because someone else complained about them. In some games people argue and if you ask nice they mute each other and start focusing on the game. Is there behavior you're having in game that feeds into arguments or does not discourage them?
Finally, I want to say that there are two other considerations. If I jump on to play a game because I'm stressed with school or had overtime at work the same week of exams and someone says something negative while I'm primed to be reactive as it is, I will probably snap at them and it may start an argument. To what degree do you punish (by denying rewards they would otherwise earn) generally positive players for acting out in a single game or two when they are having a bad day? The obvious solution is to insta mute the whole team so you don't get upset with someone because you're stressed. However, if the team talks strategy in chat and no one has been negative but you don't respond because of mute, is it justified to report you for refusing to communicate and collaborate with an entirely positive team? Should you lose rewards for this, or not, and how do you prevent loss if you think the person shouldn't lose rewards? (I.E. what distinguishes someone muting to avoid taking their bad mood out on the team, and someone muting because they don't want to interact and are actually less positive and enjoyable to play with since they don't want to work with the team?)
