honestly, everyone is able to play just fine, its the fact that the first team to have an emotional team-wide rampage will ALWAYS LOSE.
If everyone on your team is happy, then you will win. It is all about outlasting the rage. I've gotten to d5, had smurfs that i put in bronze, was extremely hard to climb up until I realized why it was so hard to win the games regardless of how far ahead I was. I STARTED FORCING MY TEAM TO BE HAPPY! Then I took a b3 account to silver 2 in a weekend, skipped b2, skipped s4 and 3.\
Thats the difference between b5 and high elo. Skill is a big one of course, but the most IMPORTANT difference is the composure, and mindset, and mentality while playing the games. People in bronze may have shit mechanics and terrible item builds and don't ward and have covered their minimap with 6 layers of duct tape, but you can accidently win if you just stay composed.