So how exactly did you want your team to deal with the Rengar? Even if they found the way to fix that problem, he accomplished his mission, you had only single target damage remaining after he kills you.
Your ult is gone as you're dead. Rengar is still in your back line killing the only champion left there, Karma, who has literally no hope of surviving against the Rengar. He likely kills Karma within seconds after killing you. It's now a 3v5, you guys lose the fight anyway.
What's better is that you're entire team is squishy, so Fizz can have a field day shredding your "front line" to bits with minimal effort. They had a tank, so while their early game was stronger than yours, they also outscale you as well.
All the peel in the world would not have won you that game, killing Rengar after he instagibs you wouldn't have won you that game.
Their team came out of laning phase stronger, your Jax was caster minion status, they had plenty of CC to deal with your Yi, and your fizz was irrelevant, especially compared to theirs. The only saving grace you had is that you had more teamfight impact than their Twitch from what damage to champions was saying, however, my guess is that everyone was dead before he could even lay damage down.
So as soon as Rengar realized it was his job to eliminate you from the possibility of ulting in a teamfight, the game was basically over.