"90% of games are winnable so long as you can carry"

So I read that, if you know how to carry, you should be able to win almost every game. You can never justifiably say that you lost because your team was weak; the onus is on you to carry - and if you can't carry, then start learnin'.

I like the concept of this, as a jungle main. I really want to believe it. But...

Let's say I am Jungling as Yi, with the following team comps

Our Panth vs Riven Our Xayah & Rakan vs Thresh and Caitlin Our Taliyah vs Yasuo Me Jg (Yi) vs Rammus Jg

I gank for Panth successfully a bunch of times, negate Riven's early pressure, and gank for Taliyah as well. Successfully counter jungle Rammus, essentially 'beat' Rammus during the laning phase. But then, when it comes to mid and late game, I struggle to see how you can 'almost always carry to victory'.

Granted I play with people ranging from mainly Silver, some Gold, and occasionally some Plat or Bronze, people seem to lose their heads all the time and I don't see how you should be able to carry them. I can be fed as Yi, and those whom I ganked for have average Kda; Panth failed to snowball despite the ganks, Taliyah just pottered along not necessarily popping off either. The Xayah who was doing well in lane is now deciding to act as front line despite 1000 pings to back off. Our Rakan wants to wander off into the jungle for no apparent reason just to get gangbanged at enemy blue. Taliyah is focussing the mastery flashing BM Thresh. How on God's earth am I supposed to 'git gud' and carry these people? Sure, a pro in my shoes would probably stand a good chance of carrying this team to victory, but that will never happen because matchmaking pits you against similar skill levels. I could probably carry against a bunch of newcomers to league. But that's not relevant, I digress.

In best case scenario in situations like these, none of my teammates have died stupid deaths: Rakan initates, their Riven and Yasuo ult into our backline, delete Xayah cuz she has bad positioning and micro ability, Taliyah runs away from the fight with 5% hp after dealing most of the damage to Caitlin, I (Yi) take out Caitlin, get peeled by Thresh or Rammus; even if I dodge their peel, it's a bit of a long shot to be able to cause enough damage against an organized and stronger team to actually win the game or the fight. I could try taking objectives, but their warding is good and they have control of map.

TL;DR How do you carry a team that is either bad or constantly makes mindless and costly moves?