It depends on what you mean by garbage. If you think it goes out of its way to make failed games then your likely wrong. If you think its got systems in place to encourage players to spend more time on it then your correct but for the most part they don't really affect the matchmaker that much.
Most likely Riot has MMR adjustments set to be a bit high. Lot of ways for this to happen. Some can be intentional some can be accidental.
but you don't need to affect win rates to change the # of matches to grind. Lower MMR changes would impact that as well. Climbing in MMR is not supposed to happen when the system has an accurate judgment on you. The entire systems supposed to have a win % not at 50% until you get there.
Directly impacting some ones win rate does change how fast they move, but simply halving the MMR gains would accomplish this as well. Remember the entire LP system is just there to hide from players when they are losing MMR, and give riot ways to adjust their system you cant accomplish in a pure MMR system. (removal of LP for dogeing but not MMR, Lp decay to encourage more frequent play, and theoretically you can have more aggressive MMR changes)
To be blunt with you even in a accurate and functional MMR system that's magically 100% fair. When you look at the time it takes the average player to climb will be forever. You never expect to see them make it to the top or out of their MMR. it would be about 50% for them and they would play until they choose to quit.
A basic, broken, brainless MMR system would treat. None of them have the same win%
1005 wins v 1000 losses the same as 105v100 or 10v5. Its a broken system because it wont count for nessesary varibales needed to make a game like league work. I Wont get into detail on that here. If it has a set MMR gain/loss (again this needs to be a variable its not a constant) then your wins and losses balance gains out and your left with a positive 5 match. It would treak the players with 2k, 200, 15 games as the exact same MMR. It would put them into a match and call it fair despite the fact a player with 15 games is much more likely to be inaccurately placed over a player at 2k games.
In more complex systems like this one you see a lot more variables impact an accounts gains. A common one would be K factor changes, with those its not just the # of games and overall win% but also milestones that adjust your gains. You can make a pretty good guess on your MMR vs current tier off your total wins to losses, rule of thumb it like by the previously mentioned brainless system and you have an estimation for if your likely above or below your starting MMR for the year Compare that to your guess and then check your LP gains are high med or low. If current tier < Starting tier, Losses > wins, Lp gains are low. Odds are your MMR is under current tier. When the flipside is true odds are your sitting above your current tier.
Anyhow point is that its a lot easier to change MMR gains losses which universally affects all players equally then it is to make a system that trys to actively help or harm individual players. Accomplish the same goal of upping playtime to see changes on the ladder but no effort in trying to find players then figure out what other players are online needed to help or hurt them.
Now imo riot does have things in their system that hurt it. But ironically its usually stuff directed at fixing player frustrations. A lot of things players like actually harm the quality of matchmaking. Duoing is my go to example of this as its popular with a lot of players who constantly Duo que but does not help make a better competitive ladder.
