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No no, huge misunderstanding in what you based your logic on(remember logic is NOT universal its highly conditional and individualistic if set parameters are not given to adjust): It works similarly to the following (and has been stated as such since I am just paraphrasing and adding some numbers for relational clarity): The match making system pairs you vs slightly better than you players (particularly in ranked, but see promos later), meaning they should be 1-20% more skilled than you, at 90% skill for instance a 1% skill increase can represent 4x more QUALITY practice(vs incorrect practice that harms your skills and reinforces errors)** than the previous 90% combined, so the "gap" seems increasingly larger the more skilled you are even tho its droping from 20% better than you to a mere 1%.**
It primarily and foremost matches you based on your win %, plat+ has 70%+ win rate per player so if you have a high win rate you will get "shifted" to be vs "equal or more skilled opponents" (getting trolled by challengers when you are only silver for instance). Once you are around 80% win rate (this is especially so if you have superb team work skills but low role skills, as it will place you much faster vs harder opponents the more "competent" you are at a team based game (tl;dr: while a soloist or top lane might get to diamond via mechanics alone, most people sitting in diamond+ actually have fairly poor mechanics(low skill) but excellent natural team work(synergy) which is also technically a mechanical skill. coupled with good old fashioned luck of getting grouped with like minded people(you are ALSO getting group with like minded people, its just your mindset and playstyle is not compatible with a team game so your groups might be like nails on chalkboard or the equivalent of an IT group of strangers vs a group of playground sports team whom all knew each other since childhood; though do note that this does NOT work out the same 4 man or less groups (premade) but does work roughly the same in 5 v 5 premade(which doesn't really exist outside of pros).
3rd During promos you are INTENTIONALLY paired with a group slightly lower than your skill(but usually in the bracket you are trying to enter!) and AGAINST a group(meaning all members on the team, vs just your heavily preferred role/lane(in the soloist example, solo top, or support, or jungle, it essentially looks at your over all statistic instead of role statistics unlike normal ranked matches) with MUCH higher skill/the logic of the algorithim is heavily favoring (probably 52-53% chance for them to win), it is working as intended that you should not progress in series (without competent help from your team mates as they are also a metric in this but it gets way wordier than this to add in all those variables) as Riot's refined match making system pairs you to have a roughly 49-51% win rate and tries very hard to pair you against opponents it THINKS (using logic) you should LOSE to, if you win, subsequent matches will get exponentially more difficult each win in the streak and your MMR will change dramatically for your promo series. On "average" you should: win 1, lose 1, win 1, then 50:50 to advance, if you win 1, win 1, win 1, promote, you either a) had excellent team mates and team work, or carried hard while micromanging the entire game. Of course you can game the match making system just as easily as it tries to exploit you and put you on a lose strict certain days. Generally if you are actually successfully climbing ladder you should be easily maintaining a 70% win rate, and not playing days you aren't.
Summed up: Your logic has a key flaw. The system is Working as Intended, you are supposed to lose your promos (according to the logic(math version btw) of the match making algorithm).
You CAN game this by intentionally abusing the algorithms logic(lose games it thinks you are supposed to win, avoid winning games too "well"), though this only works when there is a large player pool in your grouping to work with(so diminishes in effectiveness around platinum). Another good way to game the system, is: if you are on a win streak, and suddenly enter a promos, test the waters the first promo to see if you win, if you do not STOP PLAYING that day and do not resume your promo until you get the "feeling." (your intuition mentions to you "right now is a good time to play" this is a very specific feeling and not to be confused with the "I want to" feeling (which is eerily similar but is generated by your irrational parts (ego) rather than your intuitive/logic parts) despite their similarities, DEFINITELY listen if you get the "DON'T" feeling (that strong sensation like when a monster is charging at you and you are genuinely debating to fight or flee despite your body telling you fully to flee) though this might be a rare feeling if you aren't in touch with yourself/fully tested and confident in your own limitations and accepted what you can, and cannot do.)
some asides on winning vs losing: So, the system only looks at your skill performance and potential (how you act, behave, and preform. Not how you are feeling, how well nourished you are at that time, etc), there are tons of variables you can control that it can't predict for (and why you can have a 70%+ win rate). If you consistently make mistakes, the system knows it and adjusts, if you rarely make mistakes or always play flawlessly, it will be very harsh if you are ever "not feeling at peak performance." You will get the most accurate MMR if you only play when you are preforming at your peak (look how few games logged pros have on their "main" in ranked vs when they want to "climb" the ladder again on an alternate account/learn different roles), as it will always be the most honest metric of your maximum potential, playing at ANY OTHER TIME of day/day of the month will get you a highly flawed MMR that will do everything you are complaining about encountering.
**There is a lot of excellent advice in the reddit league of legends on how to not lose your promos, worrying about succeeding is probably the most crippling thing you can do/feel for it(anything really) however, but the basic gist of all the advice requires one to entirely change their mindset, perspective(s), attitude, and way of approaching things, which given the inherent flaws in the initial parameters you based the logic in your post on, as you currently have a defeatist/judging style, instead of an accepting/evaluating(introspective?) learning focused style you have a huge mental block from progressing successfully. **
If you are not interested in all that and want to trailblaze or be a genius instead of common/normal, you will have to come up with your own different ideas how to compensate for whatever is holding you back, your own unique style, rather than seeking information on forums or guides, experiment on your own and find your own way, lots of people in all the brackets playing that way too, and each uniquely successful. I would recommend simple experiments like playing at times of day you never play instead of being consistent, you might find a sweet "hour" once a week its best to play in and you win 100% of your matches.
~~A fun side bit of truth: You are only going to win, when you are going to win, (if winning it is the only thing that matters to you,) don't bother playing at other times. (this is how all "RNG" systems work btw, and has to do with determinism in physics (probability and statistics is actually hogwash, and only relevant in economics (completely doesn't apply in any other area, and still only loosely(but reliably) in low-risk investment(wager/gamble) environment of economics)
How to read action-consequence chains to determine when you will win and won't is another matter however(and while when you "won't" win is for certain, when you will win has more variables(the latter is a consequence function of your actions, while the former is related to consequence chains from a long time ago you have no control over other than being present for their events or not. For instance, you will never "win" if you never play at the times when you would (or could depending on how well you picked from your available choices(door number 1, 2, 3, Xnth#@? why underneath the floor of course!)after weighing the cost/benefit of the consequences of that action). Random Number Generators try very hard to mimic this real life determinism thing with MMRs, Secret PID Seeds on account creation, username choices, etc, but they all fall mercilessly into highly predictable patterns unless the same RNG is used universally for all players without error or adjustment (which would mean when anyone wins, everyone with them would also win, etc) but also why things like "personal loot" is desirable (a private RNG JUST for you, which could be great, or balls luck depending on your "luck").
(clarity: in league, except during promos, it displays how many points you lose/win in a match: If you lose a lot more points than the average number riot gave: You lost a match the system's logic thought you would win (a very bad thing for your LP), if you win very few points (4-6) you crushed it in a game the system already thought you would win, if you lose/gain exactly 10 points you are playing exactly as the system predicts for your with its logic and are not improving or regressing (meaning if you keep up that play consistently you will win 51% of your games and slowly climb, but see ALL the above on how that gets partially thrown out for your promos specifically).~~
Ultra tl;dr: The game is sided to be intentionally more difficult than your current skills because a Homo Sapiens Sapiens cannot increase their skill unless the task is at least slightly challenging (and worse if it is not challenging you may actually encounter skill regression, depending on which brain type you have). Easiest way to win your promos is A) Change your Attitude or B) Take a break and don't come back to complete them till the "right" time.
