I am at a point where I think this coinflip matchmaking is totally intended by Riot
I mean... you can't tell me that they don't see this.
I have a conspiracy theory:
What IF... they realized, that a game where you have less control about winning or losing makes you even more addicted than a game where you can just climb to your skill-cap Elo and be happy. Think about it! Why are so many people addicted to gambling? LoL actually IS like gambling nowadays!
Atm it's like: "ok i just got these 2 unranked feeders in my team and the enemy team the Master Smurf, that's the reason i lost. Next game the enemy's will get the unranked feeders and I will come closer to my goal!"
Every company this size is just a huge money printing million dollar factory and they will do EVERYTHING to make people play even more and more and more... 100% they have people hired who are specialized on this shit. The same people who invented the lootbox system for other games.
I don't know how it is for you but in my case the queue time went down from 1:40 in season 8 to 0-30 seconds in season 9. Another indicator. It just has to go fast. Shit on quality of matches, just fast, fast. Faster in the next game! Don't give the player time to think about a break or stop playing.
I can't imagine it's all just coincidence. I bet smart people at Riot used a lot of time, brainpower and statistics to construct this cancer. Maybe they saw how they lose players to other games and decided to go more drastically now.
I thought about what would I do if I would reach Dia4 (atm Plat1/2 peaked in Dia Promos several times)... I guess i would play a bit less because I reached my goal. I even told my friend who wants to play new Civilisation Add-on Gathering Storm with me since weeks: "yeah let me first hit Dia, than I will make a LoL break and we can play all day long".
And then I find myself in a weird match against 80% wr Smurfs together with first time champs, saying to myself "yeah ok... One of these games again... BUT NEXT GAME WILL BE BALANCED FOR SURE!"
Edit: I don't want to say it's all about luck. I am not one of those guys who thinks he deserve Challenger and it's just the system which holds me back. But you can't deny that matchmaking is pretty fucked up atm. And if you don't play an 1v9 champ than it's pretty hard to climb the last steps to your skill cap.