Ok, so, this isn't rank shamming, so please read to understand, not just to reply, ok?
You are Silver, and as we all know, at least 70% of the LoL population is between Bronze and Gold. That means you have a lot of different types of players in those. People who want to learn, people who care to help even if they aren't the best, and also the dumb dumbs.
Now, you say that you always have competent players when you win, but idiots when you lose, and this to me looks a lot like confirmation bias. What I mean by that, is that you, along many others, look for mistakes in the wrong place, that being your team. On average, a team that gets ahead will make less mistakes than one that is behind, hence why "when you have win streaks you have good players, but when you are on losing streaks you have the idiots".
Due to high variety of players, you must not forget that there are many players who have a good grasp of the game, strong mechanics and knowledge....but only when they are ahead, as they have problems when they are behind. Meaning that if he gets ganked a few time early, all his game plan is set in ruin and he doesn't know what to do anymore, on the other hand, if his jungler ganks, or he gets ahead in lane, he will play that match like he's way above his current rank. Most players in Bronze/Silver/Gold and even Platinum are like that, hence why the misconception of "I always get noobs who lose me games". They aren't bad per see, there are still aspects of the game that they haven't mastered yet, hence why they are still in the rank the are. Same goes for you.
Another thing to point out is team calm, due to the above mentioned misconception, and many other facts, games can be decided at first blood, especially if the enemy scores it, based on who talks first. If you have a team mate who rage qued another game, and someone else in your team dies as first blood, he will get tilted, and maybe even flame, so before he has a chance to even write, you have to type in ASAP "it's cool, mistakes happen, we got this", even if the one who died doesn't type a "mb" or "sorry" or what not.
Me and a friend have decent winrates, 50%+, mine currently being at 54% in ranked (Silver as well, need 2-3 wins in a row to get Silver 2, but due to the forced lower start, I started Bronze this season), and we never expect good team mates. That's something important. Never expect. There was one game where we actually got 3 team mates who were actually good. I got ganked by Diana jg vs Malz mid, Yi tried to help, but sadly for him, got there too late, so I just took the free double kill, he then went Herald and gave it to me, after a gank top, Darius allowed me to get the first turret kill, by that I mean, creeps die, the turret needed 2-3 autos to die, so he stopped hitting it and tanked it so I can take it, and while my friend as a support went roaming, Lucian didn't cry that he's left alone in bot (My friend is support main, and almost always the adc he gets randomly will cry when he goes roaming). We acted like we're awe struck because we had good team mates.
With that I try to say that, when we start a game, I expect to get camped by the enemy jungler while my jungler gets executed by Rift Scuttle, my top/mid (the other lane, depending on which I am) will not know how to play his champ, and the adc will cry on every little thing. By having so low expectations, we don't get tilted when we actually get said player, because we always have a plan on how to make it work based on the team mates we have, but when we get someone who's better than the "expectations" we have, we know that the game will be much easier to play.
Tanks that are supposed to initiate but sleep in the back line? Had it. First timers? Nothing new. Trolls? Yes. Score board players? Don't remind me.
We had all types of noobs in the team, and because we expect them to be idiots, while the enemy team has the good players, we also improve. My laning in mid got better on champs I play a lot and I die so little to ganks because I expect to get ganked and have no help, so I adjust for exactly that. If I do get the help, that's fine, if not, I am prepared.
Expectations play an important role in your mentality, don't expect good players in your team, stop looking at their mistakes and focus on the mistakes your enemy laner made, and you will win much more than you lose. The only common thing in all your games is you.
There is no forced 50%, it just takes the mental power to accept that and stop giving your opponent free wins. No rage que, no "I can't end the day on a loss", no flaming. Support your team by calming them and lead them properly, as the difference between a leader and a boss is the same as the one between a Silver and a Diamond 2 player.