Plat splitpushes at enemy tower when they have baron?

How do you get to plat doing this shit? I said to team, "wait out baron", "then you are too deep!" but of course my team feeds. Like my team will feed whether i tell them to back off or not, it's frustrating. Fighting a 3v5 was the only reason they even had baron. Later my plat teammate splitpushes bot when the enemy team are at baron, my ward was destroyed and no one else would ward baron, and i assumed he had an idea where to be so I tried to catch their syndra with sion ult but just missed... The player had about 800 matches so maybe it was spamming ranked games, I don't know...

But leagues ranked system needs an overhaul, I have a player be the sole reason we lose a match, look up him on opgg and he is as high a rank as me! But I am genuinly expecting the player to be unranked or atleast in silver.