Let's talk about "Forced 50% Winrate"
What that means is that the more you win, the higher level games you get put into. Your MMR increases each time you win, and will continue to do so unless you lose (for that game it goes down). As you start playing against better opponents your winrate will go down, until it stabilises at around 50%. That's what the term means.
I see a lot "Oh, match making wants me to lose this game because I've won some".
No. Your opponents get better, your teammates get better. At the same rate.
Why do you have to play against plats/dia when you're gold? Because you have high plat/dia MMR and so those are the opponents the system thinks you can beat. You'll climb in that situation with a 50% winrate because that's how LP/gains losses work. Or are at least supposed to work, bugs with positional matchmaking LP gains notwithstanding.
Match-making is trying to put you in games at your skill level. You'll probably find that a lot of you have games just like the ones you complain about your team having, or are making other critical errors in your gameplay and just don't want to admit that the idiot that is 0/10 at 10 minutes into Nasus is the same skill level as you.
MMR isn't perfect and could definitely do with being visible. Invisible match-making systems make people suspicious - especially in team-based games when their teammates don't do well every game.