Your reasoning is flawed.
Just because you dont understand the reason for their decisions does not mean you and them are on the same level. Also i'm not sure why you are expecting everyone to play perfectly and alot of these mistakes are based on hind sight. That rengar may have been looking for the enemy Vi, he might have been placed off-role, or he might not play rengar very often.
"Rengar goes into ult, walks around and doesn't do anything while enemy Vi is going godlike."
This statement proves you are a gold player, vi is one of rengars counters and when a jungler gets a lead its really hard to gank because you are risking getting counter ganked and instantly losing the game. Rengar was incapable of getting vision on the enemy jungler because that jungler had complete control of the game. Without knowledge of the enemy junglers location means that ganks become impossible. He most likely thought that the only way to stay in the game was to farm it out and try to keep up with vi as best as he can.
Now I did not see this game so I can only make these kind of assumptions. Also at higher elo's, having a bigger gold lead does not mean games are easier to close out because at this elo people know how to stall out games better than in gold.
Rather than trying to justify that you are better than diamond players based on assumptions, try to understand the difference between you and them. Look at their build paths, the way they control the waves, how they trade and take advantage of levels and gold differences. Try to be in their head and ask what you would do in that situation and see what they do.
