i think that the MMR is just a load of c**p.
lets put it simple: you are S1, you play vs S1, If your skill is lower you will lose more than win, you will go down, if you win more than you lose then your skill is higher and you will go up. but you always play vs players in the same division as you, this will result in a mid/long term adjustment of the skill per division, may take a full season for ppl to be placed under the correct division, or it can take alot less considering the average amount of tanked games done per player.
now lets put mmr into this: you are silver 1 and get carried 7/8 games in a row, which isnt that uncommon at low mmr, ur mmr goes up u are matched against some g5/4 players instead of still play vs S1 only, you lose your lane since thir skill is eventually higher, their get fed due to the diference in skill, and their 5 gold players proceed to win vs your 4 gold players and a booster silver. result? 4 gold players lost mmr to the mmr system, but the positive thing is that the system will adjust the silver player mmr...
does this system have any logic? no, it doesnt, since regardless of your division (which is the ultimate thing that you get to see and compare) your matches will never be against persons within that same ultimate comparison badge but instead against ppl that share a hidden factor you cant measure and you cant see and leads to rly weird diferences in skill inside the same team because the average mmr of your team matches the average mmr of the oposing team. in fact, if u see the tools from lolking/op.gg and anything like that to check your team before game start its very very normal to see full tier discrepancies inside teams, most of the time leading to the team with less discrepancies to win (aka your team having s1, s2, g1, g2, b1, and oposing team having s1, s1, s1, s1, s1 for example. usualy the team with the low end will lose it on the back of that b1 player be so much less skiled than the g2 player in the oposite lane).
also the fact that during champion select you cant see division/tier from your team and oposing team doesnt allow your team to adjust and put the lower skilled players in the same lanes against their lower skilled players, leading to the random fator of random 0-8 lanes that fed that jax on the other side that leaded to a 15m loss... which in turn cause skilled players to lose mmr, be paired against lower skill player and keep this system going.
now, IF you only played inside your division, it would take longer for you to go up a division and tier, but you will always be playing against the same skill players.
lets imagine you get 5 lp per win, 5 per loss inside divisions, this would still alow for a 5 game winning streak to start winning 10 lp's instead of 5, and thus a more skilled player will still go up as fast as they would, and reach their ideal division/tier. currently you get high MMR, you get paired against same mmr, and you will be winning at a 50% win ratio more sooner than your intended division/tier most of the time. unless ofc your skill is so much diferent, like you start s2 but your a high plat player, in that case you will climb close to platinium untill ur mmr puts your win % near the 50% but even so your win % stanbilizes under your actual skill. if your skill is a gold 3 one, and you are in silver 1, then by the time you reach gold 4 your probably already at the 50% win ratio, making it rly hard for your division to actually state your real skill level and division, since your not playing against your division, but against your mmr, regardless if you re silver or gold.
having a division/tier system, that is only for show because it doesnt reflect your actual skill isnt good, specially one that doesnt take into account your individual play inside the game, but instead if your team wins or loses.
the game calculates the mmr based on your team performance, thus reducing the chance of mmr being right by ALOT. making the chance for mmr to be near 100% right at more games than most player actually play in a season.
the division only system isnt by any means perfect, but at least doesnt put in a team that has such levels of skills inside it every single season, after one season you should be placed inside your division already, and all division should be stable, consider it the SEASON ZERO. the enxt seasons will start from the division you were last season, allowing your skill to have grown and still be able to go up, or less skill and go down. whereas atm your new season start with ur MMR reseting to a average of the original mmr every1 starts with the mmr you achieved last season, making you start much lower and witha higher possibility that players lower than you got carried some placement matches higher than they should and actually lose you more placement matches than you would normally do.
also 10 placement matches arent a real indicator of the skill, considering it take 100 to 150 games for the mmr to be adjusted properlly most likelly, making ppl start in higher skill mmr or lower skill mmr, leading to even more and more of this cases. in the end the mmr doesnt reflect your skill level, but your luck to play with ppl from higher skill level.