MMR is a load of crap

So recently my main account has been hovering between gold 5 and gold 4. For some reason, if I lose one game at Gold 4 0 LP, I get immediately demoted and this has happened multiple times. I've checked my mmr, it's normal for a gold 4 player, yet it always only takes me one loss to get demoted. Besides that, as I understand it, your mmr is supposed to dictate how good your teammates are and how good your opponents are. If your mmr is lower, you get slightly better teammates, and vice versa if you have an above average mmr, or at least that's how it's supposed to be. I recently had a game where I had already lost 3 in a row, I get into this game, and the enemy mid laner (Vlad) is absolutely stomping us. After the game, I look him up on na.op.gg, and this fucker has like an 80% winrate in preseason and is so obviously a smurf that I couldn't understand how he got into a game opposite me, when if the mmr system has any logic to it, he should've been on my team. Evidently rito needs to work on their matchmaking system and their mmr system, because these things should not be happening.