You can't have it both ways with ranked divisions Riot.
The current ranked system is basically an attempt to keep a pure numeric ranking while also having clear cut divisions, but it fails miserably at this. The reason is fairly simple - it wants to have divisions with the whole promo system and also be bands of rough numeric ranges, and these aren't compatible approaches.
The reason for this is promo series, especially the 3-game series.
Matchmaking groups purely by your numeric ranking without regard for division. This of course produces roughly 50/50 outcomes in the long run once you numeric ranking is accurate. The problem is that when you're off a ways from the equivalent division that you're being given 50/50 matchups with people from a few divisions up and have to get better than 50/50 results in order to complete a series (and the reason for the 3-game series being worse is that it takes better results than a 5-game, and all else equal... it's easier to get 3 heads out of 5 than 2 out of 3). Additionally, series defeat the LP weighting that otherwise happens (why people will get 30 LP on wins and only lose 10 LP on a loss when the system thinks they should be higher by rating). This is easy to see in that while normally someone might break even on LP or better on a 1-2 record they end up down a decent chunk of LP for no reason other than that those games took place in a series.
To fix this, I only really see a few options: Pure LP based divisions - Remove promo series entirely. Let the higher LP for wins than losses move people to the correct division without the frustration of a lost series having abnormally large setbacks. Maintain LP changes in series - If people's LP change from a series is solely the result of the normal game outcomes, someone who the system believes should move up (anyone getting even 50% more LP on win than lost on loss) will immediately have another promo series (or be one win out even after a loss afterwards) if they lose 1-2, and a 0-2 loss will leave them within one win or so (roughly). Scrap rating based matchmaking - Match people solely by division and have LP change be based on the difference in current vs. expected division (actually, this is basically what Splatoon does). I don't like this option as much, but it does let people move up easier... at the cost of everyone else's enjoyment as they stomp their way through opponents who aren't on their level. Or not, because the other team can have someone like that just as easily... but over time this should sort out. I don't think it would really alleviate the issues when you're closer to where you should be though. Change series requirements - If someone is farther out from expected division, reduce the needed win rate of a series. This just lets them get through them quicker. Alternatively, each time they hit the series make it easier (or move towards that, so that if they hit five series and are rated above the next division, make it only need 1/3 or something).
I may be misunderstanding the way the LP setup works in the background some, but as a challenge to Riot - what happens if you admin a Diamond player into Bronze? Do they get through a single series any faster than if they were moved into Gold? If not, then something needs to change - series should be easier the further out you are.
tl;dr: The current matchmaking works in the long term, but promo series hurt the convergence of division and rating that eventually happens, to the detriment of player enjoyment.
