You got my upvote.
This isn't exactly how it works, but it's pretty accurate regardless. This is why you play with friends instead of solo queue. Being that you made it to plat 4, safe to say you already know how mmr and elo works, but I'll clue you in for reassurance.
Elo is where you are ranked and your mmr should reflect that, with the goal to keep teams at a 50/50 chance of winning. When you start to go on huge win streaks, your mmr increases dramatically, but your elo is still the same because you have ranked up yet.
At this point, your mmr becomes significantly higher than the average player in your rank when you get to that 53%+ win rating. To balance this out in solo queue, your teammates need to have a lower mmr, with the biggest disparity being in your promos on a win streak.
The system was made to test your mettle, if you truly deserve to rank up, jump through this obstacle.
Problem is, 9 times out of 10, the players with lower than average mmr (on the verge on being demoted) that you in end up with in your promos are trolls. They're not supposed to be trolls, just players that aren't as efficient. These matches should still be easily won with those summoners freezing their lane or just taking on a tank role and playing safe. When the system was made, trolls weren't taken into account, and RIOT implementing a new system solely for trolls management would be sending a bad message to the rest of the community.